I just got myself a set of used Comptech Springs (black) and for the most part they look great for 15k miles, but in between the coils there is minor rusting. Is this normal?

It is nothing like the silver rusting threads I have seen, but its a little.

You gotta look closely:

 

The black ones are the new ones. I"d clean them up and see if the rust is really from the springs and not from other suspension components.

 

The rust is definitely from the springs. I tried cleaning them up and it seems the rust is between the coils of the rear springs. I wasn't sure if over time this is normal or that it shouldn't rust at all...

 

Technically it shouldn't rust at all. If someone was rough handling the springs they could have chipped the powdercoating.

Unfortunately since you bought them used you have no recourse with Comptech. They will only exchange them if you are the original owner.