An old joke in the Engineering field is to send the newbie to go find cable stretchers on the manufacturing floor. Someone searching for black chem-film can feel just like that poor newbie. Chem-film goes by other names, such as chemical conversion coating, iridite, or alodine (some of which are trademarked terms). However, putting the word black in front of any of those will provide very disappointing results.
Chem-film is sometimes used instead of anodizing as a protective finish for aluminum. Chem-film leaves a coating that is conductive to electricity. Though an anodized surface may also be conductive, chem-film is more so.
A chem-film finish is typically yellowish or gold in appearance on aluminum, but may also be brown, gray or blue (depending on the substrate). This is not the result of dyes. Lighter processing can result in a clear finish. Either way, the surface color comes from variations within the process itself, and not the result of any coloring additives. Factors such as the reaction of the substrate with chromic acid, temperature, inhibitors, concentration, promoters, time, surface finish, and accelerators all play a role in the final surface color. This contrasts with anodizing, which may be dyed to intentionally achieve a variety of colors.
Be cautious of a vendor that says it can provide black chem-film. They may be using the term chem-film very loosely (i.e., incorrectly). That said, if a vendor can legitimately produce a black chem-film coating on an aluminum substrate, they are invited to comment on this article. Of course, I will ask that verification to any such claims be included within the comment, along with confirmation that results are compliant with MIL-DTL-5541F other similar standards.
On a side note, there may actually be black conductive coatings available for aluminum. If I can get confirmation, I’ll mention those at a later date.
Reference: MIL-DTL-5541F (.pdf)