Armpit bruises from crutches?

If there is no pain in your arm pits then it is more likely that you have built up a callous there. The skin has been rubbed and so it responds by getting thicker and rougher. The discoloration is just a side affect of your skin getting thicker there, so it can stand up to the repeated use of the crutches. It is a natural side effect and should go away a while after you stop using the crutches. It might take some time and it might not fully go away, but it is a side affect of the skin being abused by the crutches.