Anodising is the way to go but finding a 'friendly' anodising place willing to take it on is the problem I struck. I tried a few places but they either priced it way to high or said they wouldn't do it for various reasons. You could set up an anodising bath yourself as I thought of that as my rail (also aluminium Llagas Creek) is cut in 3 foot lengths so not needing a six foot long chemical bath for 'normal' lengths of rail and it only needs low voltage for the bath to work so it can be done.
But in the end I just painted it, this is not ideal as 7 years on where the sun hit's it the paint has faded but much better than plain aluminium colour.
Use a good etch primer on clean, scrubbed aluminium before applying colour if you go the paint way but see what you can do with anodising as that's permanent and should look great.
I have the same rail and find dark brown paint worked great and leaves a steel looking railhead. I dont know of any rail I've seen in the field that is really "black." Black is the total absence of color, and when used in models is distracting because it is not a color found often in nature. Most things we consider black are really dark gray or brown. the opposite condition occurs with a pure white such as titanium white. It's good for mixing, but as a stand alone color, it usually creates issues.
I'll have to look into this sometime soon. Typically customers use paint to model weathered rail, but it's good to know some are interested in non-paint methods. The downside to anodizing or dying is that the color layer is typically very thin, so even light handling of aluminum rail can scratch through the thin color layer. I imagine even just sliding on the tie strips will scratch and mess with the color detail on the rail foot since our strips hug the foot pretty snug.
In a nutshell; I made snakes of steel wool and clothes pinned them to the rails. I sprayed with accelerators (hydrogen per oxide, acids and water (rain water was fastest) and let the steel rust.
The tricky part was sealing the rust powder on the rails. I finally found a spray acrylic that I could mist on instead of blowing the rust away.
Took about a year to do 150' of track.
I have a new method in mind, that should speed it up considerably, but health issues slow me down.