I just had the same experience! I'm doing a red/white/black quilt and on prewashing the fabrics, the black bled a lot, just like your's did.
The LQS owner told me black is bad for that due to all the dye they have to use to achieve true black. She recommended adding white vinegar to the water and it sets the color. I tried this on some extra same black fabric I had to buy and it worked! But what if I hadn't pre-rinsed it first and had made the quilt, then washed! Scary. I have used the color catchers but I understand it would have to be used each time it's washed and what if the quilt recipient doesn't do it? I know red bleeds but it wasn't nearly as much as the black. Thanks for your post!