Traveling to Germany before using a type D visa for Belgium

I see no reason why you could not apply for a short-stay visa from Germany. I am not entirely sure of its legal basis but everything I found on the rule that forbids people from holding several Schengen visas is along the lines of “a person cannot hold two uniform visas valid for the same period in time” (e.g. this particular phrase is from page 26 of the Schengen visa Handbook). I think that your Belgian visa is not a uniform visa (which is Schengen jargon for a short-stay visa valid in the whole Schengen area) so you could still apply for one.

That said, I don't think that it is necessary either. A valid long-stay visa from a Schengen member state gives you the right to travel for up to three months to other Schengen member states (article 5 of the Schengen Borders Code). So as long as your Belgian visa is already valid, it should be enough to enter Germany. You do need to fulfill the usual conditions for a short stay in the Schengen area (i.e. justify the purpose of the stay, have travel insurance and sufficient financial means, etc.)

Beware: Some countries issue visas that need to be “validated” after you established residence (e.g. you need to complete various formalities within 3 months of entering the country and then get a sticker in your passport). The initial visa is valid to enter the country the first time (perhaps also to transit in the Schengen area?) but I am not sure if it would be considered a valid long-stay visa in other member states or if getting an entry stamp would start the three-month validation period (I also have no idea if Belgium does something like that, so it might not be relevant to your situation at all).