What Causes Foot To Turn Blue?

Hi, may I answer your health queries right now ? Please type: my left foot turns blue when I stand and put pressure on it. First noticed it in the shower about 6 months ago. I have had circulation checked. I was laying down at the time and was told my circulation is very good. No other problems like diabetes, blood pressure. 71 years old, 5'5", 170 lbs. I suppose I should go back and have it checked while standing... would still like your opinion. Thanks