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Are you able to take a picture and send it in quickly?
If not, please reply with how long it has been black.
Does he have something obviously cutting the circulation off?
Does it hurt?
Does he have a pulse in the wrist (can be felt on the side below the thumb on the wrist area).
Does he appear to have circulation in his fingernails? For example are they pink? If you slightly press the tip of the nail does it become white and quickly red again when released?
I'll look for your reply.
It is possible that it doesn't hurt because it is numnb.
It is important that he be evaluated. Unless this is topical (something black has gotten on his skin) this means his circulation is obstructed to part of the tissue. It may be that he has small clots that have cut off the circulation. Especially if this continues he could lose the tissue, or it may be that clots could occur elsewhere even causing a stroke.
I'm not sure what they are called in the UK but you do need to take him to an urgent care clinic or an evaluation. This could be a very serious sign.
Feel free to respond.