There are a lot of different temperatures that make a burn. What I mean is the burn caused by a red-hot pin is way more intense than one that has been in the flame for a few seconds. I have found that there is a range that can satisfy an SI urge for me, without leaving a long-lasting mark. Even a 2nd degree burn (a blister), if it is small is not noticeable like a cut is.

As for care, if the blister is intact, you dont really need to do anything. If it opens, keep it clean.