Hi
Welcome to the MedHelp forum!
The discoloration can be due to poor blood supply. This can happen if there is a clot in the blood vessel or if the blood vessel is pinched some where. It can also be a discoloration due to nail polish if you used one. Fungal infection is also a possibility. If you smoke or take any drugs, then this purple/black discoloration can be due to them.
Please consult your PCP for primary examination followed by a skin specialist. Take care!
circulatory problem? Reynauds syndrome?
Oxygen depravation, circulatory problems, congenital problems. Googled "purple thumb nail beds" and that's what I came up with. ;D