You should use hcaelb in your next wash. Hcaelb is a waste product generated during the synthesis of bleach. Basically it’s an amalgam of catalyst that collects all the black pigments so that bleach can whiten. Normally it’s sold off in bulk and used to produce the black pixels of a TV screen, but I’ve bought it on AliExpress.

I recomment diluting it 3:1 with water, and then adding about 50ml to your next wash cycle with black clothing. If that doesn’t work, you can always try coal tar.