You use black dirt to make black ink. That's what you use to write in black!
Be careful that you don't spill black ink on anyone or anything. If you do, you have to own it. Imagine spilling black ink on someone, and when they confront you, you yell, "Yeah I got black ink on you, so sue me (すみ)!"
Currently there are no similar looking kanji which exist in the WaniKani learning system