If you're identifying color for pretty much anything digital, you're working in an RGB colorspace. If the project you're working on requires percentage representation, black is made of 0% red, 0% green, and 0% blue.
If you're identifying color for a print project, you're most likely using a CMYK colorspace—the percentages are 0% cyan, 0% magenta, 0% yellow, 100% black.