If they're heat shields, you can have them ceramic coated. Shouldn't cost more than a couple hundred bucks. The other alternative is to try some high temp matte black or satin (whichever is the closest match) engine paint. It won't be perfect, but it'll make it less obvious. I suggest cutting a perfect circle out of a paper shopping bag about an 1/8" in diameter larger than the blemish, tape that to the exhaust (over he blemish) and spray two light coats. Carefully remove the bag while the paint is wet.