Why could your hands turn blue out of no where?

Raynaud's phenomena. The color changes you are describing are probably due to spasm of the small arteries in your fingers. Sometimes fingers will turn white when the spasm begins, then blue, and then red when the spasm resolves. Many patients don't seem to have all three color changes. Raynaud's is usually a harmless finding (primary raynaud's), but is occasionally associated with other diseases.
Blood flow. A problem with blood flow can cause discoloration in hands. Would recommend evaluation by your doctor.