I dye wool and have some experience with dye. I have black Venga rock pants that i got spatters of bleach on. I dyed them using high quality cotton dye in my special dying pot… using the whole dye process that has always worked. The dye held for a month. Then I tried the Rit dye you can get at the grocery store. Lasted two washes. Then sharpie, one wash. Then sharpie fabric. One wash. Then copic marker, one wash.
I do not know how to get my venga rock pants black again and they are my favorite and I wear them literally every day and it makes me so sad.