It's similar to being electrocuted, the marks would be the result of the energy moving through skin. Like being struck by lightening, only it's all inside your character at once, and then the creature takes it back.

Cutting, scarring, etc. will have the same effect on it as it would regular skin.

The marks are mostly just scars, the body's reaction to the energy feeding. The sample would reveal that the tissue is damaged, darkened, but is just as healthy as undamaged skin.