If I were you, I would check my drinking water for nitrates. They are responsible for "blue baby syndrome", where skin turns blue, but I don't know about fingernails.
nail polish tehe
Family Feud answers: nail polish, cold, dead, bruised
blue fingernails
Blue fingernails should look fungi infection
Yes, it can.
Not really. Blue fingernails can be a sign of hypoxia, a lack of oxygen in the blood. This can happen at high altitude such as mountain climbing or flying in a depressurized airplane higher than about 12,000 feet altitude.
blue w/ black crackle
The character of Kate Roberts like to take fashion steps to make herself stand out, such as the black fingernails or the blue streak in her hair.
The white area in fingernails and hair are caused by air. That's why snow is white and not blue. Little bumps and bruises that are done to the bed of the nail cause these imperfections.
alice's nails were why do you care?! red? blue? pink? green? orange?
A purple or blue. Metallic colors look cool too!
Well, the actual nail is always dead, just like your hair. The blue comes from something like a bruise in the skin underneath the nail.
penta-fingernails
you duct tape their fingernails and when they forget that the pick their fingernails and then its all done