It is not likely to upset your blood levels. Magnesium is lost thru the kidneys. The most likely way to poison yourself is with enemas made from magnesium salts. These then cross the membranes of the colon, in high concentration.

Only a small amount is absorbed thru the skin. I think most remains in the tissues that are soaked and does not move around quickly. Most people are quite low in magnesium intake from foods.

When you soak in a bathtub, use only about 4-6 oz in 6 inches of water. I don't use a full cup as suggested, because less works just as well.