I think what has happened is a reaction between the aluminum and a "base" -- high pH -- alkaline cleaner.
Many years ago, when my '72 Stella TDF was just about a year old, I labored for one summer on the swing shift cleanup crew at a hotdog / sausage / ham processing plant.
If we were cleaning stainless steel, we mixed up a "hot" bucket of caustic soda -- sodium hydroxide -- and water, and it got things really clean.
If, however, that solution was used to soak anything aluminum, the aluminum would turn black.
For aluminum, we used phosphoric acid and a green scratch pad.
I doubt that your cleaner was as alkaline as the caustic soda mix, but, soaking overnight would have had enough contact time to turn it black.
As I recall, a lot of time with the phosphoric and scratch pad were required to get the black off. It was the kind of mistake you only made once.
Vinegar, and maybe one of the orange-oil cleaners, might work to get the black off. You can probably find a surface (on one of the arms where they overlap) to test.
Joel Niemi
Snohomish, Washington, USA