Richard,

Definitely mention this to the specialist on Monday. I've noticed the same thing a couple of different times, where the vein network at the base of my thumb (in the fat and muscle pad there) turns a dark blue, almost black blue, as it is filling with an unusual amount of blood. When that happens, the rest of my hand often looks a dusky purple and some veins distend painfully, though not always. Because the nerves and blood vessels all run through the same "outlets," you can have compression of both going on at the same time. Sleeping on the affected side can cause further constriction for multiple reasons, so if that was the aggravating factor, I would try not sleeping on that side for a while to see whether you remain symptomatic or not. Again, definitely mention it to your doctor, because a blue hand, and at the very worst a blue limb, often mean there is venous constriction in the thoracic outlet.