They have been used historically for lethal effect for many centuries and quite successfully. What kind of black powder are you thinking of? Rifles or revolvers.
One advantage with black powder is a rather large projectile and being round instead of pointy, it creates massive damage at close range.
I wouldn't use a pirate flintlock but a late western revolver or rifle using cap and ball would probably work, I would certainly test fire it before hand on targets to be sure.