I have used Aluminum Black on scope parts with limited success.
I repaired, cleaned, and reblued a steel tube scope for a customer once.
The tube was all steel but the adjustment saddles were aluminum.
The steel blued up nicely. I used Aluminum Black on the saddles. The aluminum black works by oxidizing the aluminum, leaving a black layer of oxidation kind of like what you would see on an oxidized aluminum truck tool box.
The oxidized black layer rubbed right off of the aluminum saddle with a rub of the finger.
Thankfully the scope looked really good with the blued tube and bright aluminum saddles and the customer liked it that way.
The black marker idea is a good one or maybe try a black paint-pen you get at a crafts store.
JB
