I don't really have time to read everyone's posts here so forgive me if it's already been said...

First off, that's a female eastern garter (T. sirtalis sirtalis). The scale blemishes look to me like the very early stages of scale rot. I don't mean those marks down the center of the snake. That's usually nothing to worry about. Lots of snakes have that. I mean the brownish disfigured scales. I would completely change out the substrate for fresh clean new stuff. Before putting new stuff in, disinfect the tank and everything in it.

Scale rot is caused by bacteria that flourishes in damp, dirty substrate and/or constant high humidity. It can also be caused by the snake being in contact with it's own feces, perhaps in a dirty damp hide. Wild snakes sometimes get this during brumation. Keep the tank clean and dry, and keep humidity moderate. That should keep it from getting worse but it won't correct blemishes that are already there.

In some cases, usually advanced ones, treating the snake with a strong antiseptic such as iodine or chlorhexidine may be necessary.