Heathrow Customs - "Arrivals from the EU" exit

The blue channel is for travelers from the EU who have nothing to declare. If you have something to declare you should in any case go through the red channel.

The difference between travelers coming from the EU and other travelers is that some procedures like declaring cash are unnecessary and the limits, e.g. for alcohol, are much higher than when coming from outside the EU. You can basically import just about anything for your personal use. Other EU countries do not offer separate channels, however, and for the system to work it seems you would need some way to make sure that people coming from outside the EU do not simply sneak through the EU lane.

I don't have any particular insights into the reason the UK organized things in that way but the blue channel could be a way to sort out travelers that are less likely to be importing something illegally. Baggage tags from EU airports are marked with green stripes so people who go there by mistake or try to cheat (without being too clever about it) can still be spotted easily.

The only difference I could find between the UK and some other EU countries without separate channels is that in the UK, declaring cash is only mandatory if you come from outside the EU (it's EU law) whereas in France or Germany it's also mandatory if you come from another EU country. But it's not clear to me how this could explain the blue channel.