What can cause fingernails and toenails to turn blue?

It could be...: A lot of different things, but circulation disorders will cause this. See your pcp or a vascular specialist for more detailed evaluation and treatment.

Possibilities: This could be from trauma (like bruising), vascular compromise (diminished blood flow), damaged nerves, or more specific conditions like raynaud's (usually associated with autoimmune diseases). Talk to your primary care physician first and a referral will most likely follow based on his/her findings.