Sure - Perma Blue, in any of it's incarnations, is designed for steel, not the aluminum your lower is made of. It will work on the steel where the bayonet lug used to be. Since the steel of the front sight base is parkerized, not blued it won't be a perfect match, but if you leave the cut surface somewhat rough (ie, not polished) the cold blue will be close enough. I've had better luck with Brownells cold blue products than Birchwood Casey, but if all you want it for is hiding the missing lug, its good enough.
Their Aluminum Black works okay on scratches, not so well on larger areas that need covering. I tried it, then started using a black Sharpie, and then stopped caring if my ARs are scratched. You can get the receiver reanodized, but it's not cheap, and you run the risk of small dimensionial changes to the parts.