You say you want to know how to make black fondant but that you colored buttercream black, but that it "weeps"
then you say you won'tbe doing that to the fondant so what do you need to make, I'm confused.
The best way I have read to make black fondant is by starting with chocolate fondant then slowing adding black when is a deep gray let it sit an hour or so, see how much darker it gets until you get it black enough not too black to avoid the bitter taste. Do the same thing for buttercream but to keep dark colors from bleeding into lighter colors of buttercream I add corn starch to my buttercream, it worked perfect on a clown cake I made with dark primary colors when the base of my cake was pale yellow . Good Luck