How can black travellers deal with unwanted harassment from locals in China?

There is not much you can do - those people have probably never before seen anyone black, and that is the typical reaction in that cultural environment.

Imagine you are green or blue, and you go in a bar with hill billies in a very remote region of the US. I would expect them to act quite similar - if not even more 'rude'.

Those people don't consider it rude or harassment, and they are not intentionally rude to your friends - only in your view they are. In their view, the behavior is normal - they are just completely blown away by the strangeness of the looks. When you travel to exotic locations, you have to be prepared for exotic mind sets and cultural standards.

In the old days all Westerners would get that treatment, especially outside of the areas with many foreigners. I got used to it, and found it a bit endearing (when sincere, which it often was) and much less annoying than persistent (and sometimes officially sanctioned) overcharging. Sometimes I would hear less polite terms being used, the equivalent of "hey, look there's a ******", but never with undeserved malice. In the late 1980s it was not unusual to have rustic folks from the countryside (eg. at the train station) literally mouth agape at the foreigner.

If they can't get used to it and insist on taking offense where none is intended it might be best to avoid travel to China specifically, and perhaps in general. They are more likely to think of a famous sports figure or Obama than to look down on the visitor.

Not sure if Chinese universities still host many African students, but that was one place you would see blacks in the PRC.

P.S. One thing I enjoyed in smaller cities was to go out after dusk when the lighting didn't make my foreign face all that obvious. This might not be all that useful in modern times with much brighter street lighting. Also I'm not that tall, wide or otherwise weird looking. I traveled to China with a white but very much gray-bearded colleague a few years back and he got lots of attention he found a bit much to begin with. Full beards are pretty uncommon there, as is gray hair on relatively young people.

I'm sorry to say this, but what you experience is quite typical attitude toward foreigners in mainland China, and is not specific to blacks. To be more specific:

Having their hair (and, on a few occasions, other parts of their body) touched and fondled without permission,

I'm unsure about fondling, but touching wise, there isn't a concern of "personal space" in China. If you're in queue, the person behind you would literally breathe into your ear, and if you're not staying as close to someone in front of you, someone can actually get in front of you. Bumping into someone (touching) doesn't seem to be considered rude, it is matter of life. This is common even among the locals themselves.

To avoid this you'd have to stay away from crowds and queues. Which is hard, but not impossible.

People not-so surreptitiously taking pictures with them, without permission

Happens to many tourists, who are good looking. There's no concept of asking permission to take a photo in China - people might not even speak English. And I'm sorry but as far as I know, even in US one can take photos of everyone in a public place without permission.

I don't think you can do anything to avoid this.

People pointing, giggling, and calling over friends to gawk at them

That's typical attitude toward foreigners, although not in Beijing/Shanghai. Again, can't see what you can do here.

Locals aggressively trying to make "conversation", including blocking the sidewalk, with "conversation", consisting of more gawking

That's again more typical in mainland China, especially toward tourists.

Again, based on my own experience - and I been in many places around China - nothing you mentioned here is specific to black travelers. This is how things work there. I understand you might feel uncomfortable about it, but I don't think you can change how people behave in their own country. Probably the best way to deal with this is to avoid China altogether (and India too, as all those things would be very similar there, if not worse).

I spend summers in a tier 3 city - population of about a million. I'm a white male, fairly large, a bit portly.

Everywhere I go, many people stare at me, mouth agape. Others avoid making eye contact.

This is in a neighborhood where I go every year. Most of the people know my name, and they know the family that I stay with. In fact, when I travel 3 or 4 neighborhoods away, even there people will know my name.

Even though they know me, the still stare at me, or avoid looking at me, or look when I'm not looking then quickly look away if I look back. The bold ones will show off to their friends, and take pictures with me.

I want to emphasize that this happened when I started to visit this city yearly 6 years ago, and still happens today. In fact, the extended family of my in-laws are very nervous when we are out to eat at large social gatherings.

I have tried everything to make myself more "fitting in" - ignoring it, anger, politeness. I have only found one thing that works: to joke around with those who are forward with me. But honestly, sometimes I just want to go to the grocery store and buy some yogurt without stopping every 10 feet(3 meters) to greet the multitudes of passers-by. It can be irritating.

In summary, If I'm out and about, in a small town nowadays, I'll take between 2 to 5 pictures per hour, more if my son is with me. And I am not particularly attractive.

The touching thing is a bit more difficult, because when I look around, I don't see other people touching each other. I think when a 5 year old comes over to pet my "unusually hairy arms" I find if rather endearing. I will squat down and engage them, and they will usually know a few phrases of English - "What's your name" and "How old are you". Maybe "Where are you from". Usually they will be accompanied by an overbearing grandparent. Very silly.

I have never had an adult touch me in an unusual fashion as described in your question; besides handshakes and pats on the back, I have no experience with this. I have heard of women having this problem though. I am curious if it is because I'm in North China, and South China is more touchy?

One of the more interesting parts of being in another country (or even a different part of the same country) is that the culture of that area is going to be different then what your used to.

People may have a "thing" for being touched, or not. Some areas of the world touching is taboo, others, it's normal and even polite.

The same is true for photos, talking, gathering, pointing, giggling, etc.

It can be annoying at times. But it's the way things work in the area you're in.

In the OP you mention that your traveling companions are black and that they get upset by the locals "singling" them out. Why? That's the real question. Why are they getting upset? Yes, if they were in the US and singled out like that they would have a legitimate gripe. But you're not in the US. In China, touching is more acceptable then it is here. As is taking "un-requested" photos. As to making conversation, again, very normal there. Not so sure about the hair thing, but that's probably a part of touching.

Ever walk up to a random girl, touch her hair, and tell her you like how smooth it is? Of course not. That's not acceptable behavior here in the US. In fact that's likely to get you arrested. But that's not true in all parts of the world.

Your friends are not being singled out due to racism or prejudice. They are drawing attention because they are different in a culture that celebrates conformity.

My suggestion to you, and your friends, is to leave early to get where you want. Embrace the local customs, even if it's outside your comfort zone. And be just as talkative and "touchy" back with them. If they feel your hair, feel theirs back.

Chinese people touch each other for different reasons then we do. Some don't like it, and some don't mind. But if they're touching your hair, then it's probably ok to touch theirs. And if not, I suppose they will stop.

The point is, you're in a different country, with a different culture. Trying to force the Chinese to conform to the US social customs and etiquette is far more insulting then what the people are doing to you and your friends.