Causes of purpura rash. The direct cause of purpura is blood in the skin which has leaked from blood vessels, which is what makes the little purplish-red dots or blisters to appear. This bleeding can be the result of several different causes.AIN Health Rashes Purpura Rash. Blood blisters caused by purpura. Purpura, which is also known as skin hemorrhages and blood spots, is a rash which appears as an outbreak of red dots on the skin.