yep- what he said. Bluing is controlled rusting. Aluminum does not turn into black iron oxide.
Anodizing is a combination of an electrical process and dyeing of the metal surface- generally not a home project.
For small scratches, try a Sharpie pen. For larger areas, look for a product called AlumaHyde. By Brownells, black spray lacquer for aluminum.
What we have here is... failure- to communicate.