The interior of the case or the exterior of the case?
How bad is the scratch? Pencil line thin or two inches wide and 4 inches long?
If it is just a pencil line thin scratch on the inside of the case go to a local hobby shop and get a small jar of black metal primer and a very fine thin brush. Two light coats should do it. Unless the interior is lit up like a Christmas tree it will not be noticeable.
If it is a small scratch on the exterior then it might be a bit more complicated. I'm thinking a coat of black primer and then a coat of matt black paint might do it.