It really depends on the fabric. I have had blacks that did not lose any color, and blacks that were over-dyed a lot. If you are not sure and don't want to prewash unless absolutely necessary, test the fabric. Cut a small swatch and drop it in a glass of water. Let is sit for a day and see if any dye bleeds into the water. Also, take the damp swatch and rub it against other fabrics to see if any black transfers. If the fabric fails either test, I would prewash it.
I agree that black is often like red in terms of bleeding. I have had both be non-bleeders and both be bad bleeders.