Assuming you are talking about fabric dyes, black is one of the hardest colors to create when dying fabric. This is because black is not a color on the spectrum of visible light, but rather the absence of all color. In order to create a true black dye, fabric dyers must use a combination of dyes that absorb all colors of the visible light spectrum. Even then, it can be difficult to achieve a perfect black. Some common dye combinations used to create black fabric dyes are: carbon + logwood, iron + tannin, chromium + tannin, and copper + tannin.