Plant it to grow Black Hair Dye. Pour this gunk on your head if you want your hair to be black! It may require multiple applications to achieve the desired color, or mixing with other dyes. The dye interacts with your natural hair color, so some colors may simply be impossible to achieve.
Black Hair Dye Seed price in Growtopia: buyers pay about 1 WL (median of 1 offers from community Discords, past 30 days). Live price chart and the latest buy and sell offers.