I have the same band but in bright yellow. It has also got a bit darker over the few weeks I’ve had it, and washing it at 40C in a delicates bag has got it clean of sweat etc, but has not brightened it up at all. I’ve begun to conclude that it is clothing fluff accumulated deep in the hook-and-loop webbing of the strap which, if I’m right, I don’t think there’s much chance of ever getting out.