My sister has her fingernails bluish every time she uses magnesium sulfate as bath salt? Why is that?

Haven't heard of... Haven't heard of epsom salt (mag. Sulfate) causing bluish nails. If the nails are more bluish after getting out of a warm bath into the colder air, maybe the drop in skin temperature causes a decrease in blood circulation to the fingers and nails. A primary care doctor can get the details, and check the fingers and nails to see if there is a medical issue, such as decreased circulation to fingers.