The reason why the arms become blue after blood sampling is as follows: 1, when the needle pierces the small blood vessels, the small blood vessels under the skin rupture causes the blood vessels to seep, after blood sampling, after some time, due to the increase of blood seepage will appear blue, this situation generally need not worry too much, after the blood seepage reaches a certain amount, due to the amount of blood accumulated under the skin, will compress the bleeding small blood vessels to stop bleeding by themselves, the blue will also be self after some time.