Chemical composition of the black dye used in anodizing process

A. These dyes are proprietary so no one is going to tell you their chemical composition. But there are many formulations and they are very different. One important difference is that some are ultra dark red dyes and some are ultra dark blue dyes -- and they can look very different in artificial light situations. I remember a manufacturer of audio components having to correct this issue of their black equipment being a mix of dark red and dark blue under dim stage lighting.

A. Request the MSDS for the dye from each anodizer. This info probably won't satisfy you; you are seeking proprietary information which the dye manufacturers jealously guard. The Surface Treatment of Aluminum and Its Alloys.
has a chapter on dyeing which mentions one black dye being of the anthraquinone type. The information for other dyes may aid you in identifying chemical types and their bonding within the pores of the anodic coatings.

I will speculate that some spectral variation is due to the sealing chemistry and the anodic coating itself, which contains varying amounts of sulphate.