Micro-Mark has a wide variety of chemical treatments for metals, and Birchwood Casey has several, including an aluminum blackener.
For years I've used A-West Blacken-It, available from Walthers (though out of stock at the moment), and industrial supply stores, like McMaster-Carr, have carried blackening solutions for years, but the last I looked all they had was essentially black paint. Some of the chemicals formerly used were a bit too toxic for today's consumer protection advocates.
A little Googling will no doubt uncover other sources.
BillB