ARs are usually finished* in black, while "sporter" rifles and shotguns have usually been furnished* in wood. There is now a considerable portion of "sporter" rifles and shotguns being done in black, and ARs in color, but the nickname usually refers to the AR platform in all its variety.
Another FRiend calls them "EBRs" (evil black rifles) and his local merchant as the "EBR store". He also builds and accessorizes them for fun.
*not a typo.
We keep scanning for new names for the same "black" rifle (even if it's screaming pink-and-white), and came up with MSR (modern sporting rifle).
The current generation of military assault rifles came on the scene about the same time as black plastic was the only inexpensive, strong material for stocks. Except in the USSR, where 1930s-era Bakelite (probably ripped off from us) came in a reddish-orange color. One disadvantage of black is that it is an unnatural color out in the field, and tends to draw the eye to it.