Though not mentioned in the 'industry' list above, I'm gonna give it a go anyway ... ;o)
No matter how stained it may be, a good rub with plain and simple 'Ballistol' oil restores the deep pitch black color on black anodised aluminium.
Ballistol is often referred to as weapon oil, though there are over a 1000 uses according to the Ballistol website (one of them is even the treatment of pet fur).
Every stained Hugyfot housing that is serviced gets this 'Ballistol' treatment and leaves the factory looking brand new.
- ChrigelKarrer likes this