it's not a trope. a BM is more likely to have a larger penis when compared to another male group. this doesn't mean there aren't men of other races that don't have larger penises. case and point. asian men are known for having smaller penises. yet, i spend time in the asian threads. you see many asian men with average-size penises (like WM)and larger ones. there is another thread here dedicated to men with micro penises and such.
from what i've seen, it's mostly WM. when i go to armature porn sites like Gay Hoopla (mostly white), the average WM is labelled 6 inches. some look smaller than they are labelled. if you go to a site like Black Boy Addictionz (actually men, nothing weird), the average BM is packing 8 (a number are packing more). if you visit the BM threads here or even follow BM on OF, they do have larger penises.
finally, you named nationalities -- not races. there are black people all over the world -- especially in Latin America. more slaves were taken there than brought to the u.s. also, you named a lot of majority-white countries. ever been anywhere in africa? until you visit a majority black country, you can't make a fair assessment.