This condition as you have described is suggestive of a condition called Raynauds phenomenon. Here the blood vessels become narrow during which blood flow is less and therefore oxygen is less to that area so it appears blue. Later as the blood vessels dilate , the skin changes colour again. This is a condition that can occur without any cause in which case it is called idiopathic . Sometimes this can also occur when there is some underlying disease in which case we have to do some tests. Smokers are also known to get a similar disease which is called Raynauds disease. If you get any feeling of numbness or pain during walking etc it means that the tissue affected is not getting enough blood. This can lead to serious problems so a physician usually examines for problems of blood flow. If there are no such problems then you can think of it as just another annoying problem. From the symptoms that you have described your condition appears to be a type of idiopathic Raynauds phenomenon. This is not a dangerous condition and you can manage this most of the time by keeping your feet warm. You may also get your legs examined by a physician to allay your anxiety.