Rinse the black fabric in clear water with vinegar added. The vinegar sets the dye.
I had to prepare for a tee shirt quilting class this past Saturday. We were told to rinse all of our FQ's til water was clear. Mine were fine, no fading 'til I got to the 2 1/2 yards that I plan to use for border; a light black. It kept fading and fading into the water.
Then I remembered from way back in the '60's when madras plaids were big. Instructions were to soak it in vinegar water before washing! Now why didn't I think of that sooner?? So I added about 1/3 cup of white vinegar to a pail full of rinse water and swished my black fabric in it over and over just waiting for the dye to make its de'but but hey, it came out clear!!