Apparently there are a bunch of hidden dyes, including black, that are in the vanilla files and not available for whatever reason. Someone created a mod to configure them and add them to the tutorial chest, but it seems like it should be simple enough for Larian to add them to vendors or otherwise make them available. (There's a fully modded dye that's even darker black, along with some dragon- and god-themed dyes that look really sharp, if you're on PC.)
