The receiver is aluminum. Birchwood-Casey makes an "Aluminum Black" which is similar to cold blue. This may or may not produce good results, depending on how it works for you.
You can also use a permanent marker to color the scratches.
For the rust on the steel barrel, first apply a good lube like CLP Breakfree and allow to soak a day or so to loosen the rust.
Then apply more lube and brush with a brass "toothbrush". This will remove the rust without damaging the surrounding blue like using steel wool will.
With the rust off, you can use standard cold blue to cover the bare metal, you can use a marker, or you can simply paint the barrel.