The best kept secret for touching up scratches on your AR is [url=www.brownells.com/aspx/NS/Store/ProductDetail.aspx?p=1117]Aluma-Hyde II[/url], & a small, stiff paint brush!
Properly degrease the offending scratch, shake the hell out of the Aluma-Hyde II can, heat the can under hot running water, shake again, and spray a very little bit of it into a paper cup. Carefully & lightly apply the Aluma-Hyde to scratch with brush, and then wait for a week to let it cure properly.
What you get is a touch up that is virtually identical in color (matte balck) & sheen to all black AR15s' & the Aluma-Hyde II is pretty damn resistant to bore cleaner, WD-40, CLP, and normal wear & handling ([b]much more[/b] so than touch-up pens & "Aluma Black")!!!
Give it a try. I did, and I will never go back to other methods of scratch touch up!