Michael Miller's Jet Black is the blackest black, and I think it remains true through washings. I haven't washed mine yet, so don't know that for a fact.
If you can, it would be great to buy half a yard of several blacks and test. Come to think of it, maybe I will do that at some point in the future. I would cut each one in half, label each (maybe with white embroidery?), then wash one half of each manufacturer under harsh conditions in the washing machine, afterwards comparing the washed half with the new half to see if color is retained well.
Edit: I should add that I have used Kaufman Kona black and find the hand to be unusually coarse -- sort of thick and meaty, and I found that the edges of strips seem to fray frantically. The Michael Miller Jet Black that I have has a much nicer hand and did not fray like the Kona. I do believe the Kona is sure to retain its color, however.