Some aluminum alloys will turn a dark gray if you use sodium hydroxide on them, I think the 6000 series alloys are like that. The gray coating can be removed by lightly abrading it.
As with an acid wash, spray it on a limited area, work it mechanically somehow and rinse it off that early. Don’t let it sit.
I’m cleaning up some aluminum parts that are of a Japanese 7000 series alloy, as we speak. Sodium hydroxide and a gray Scotch-Brite pad.
In progress on the left, before on the right:
