Aluminum pots will turn black in use
Usually water looks clean on the surface, but in fact, a lot of things have been dissolved in it, the most common are calcium salts, magnesium salts, followed by iron salts. Different sources of water contain more or less iron salts, and these iron salts are the "culprits" that make the aluminum pot black.
Since aluminum is more active than iron, the aluminum pot encounters water containing iron salt, and the aluminum replaces the iron, and the replaced iron attaches to the aluminum pot, and the aluminum pot turns black.

However, for this chemical "trick" to be successful, three conditions must be met:
First, the water contains more iron salts.
Second, the boiling time is longer.
Third, the aluminum pot must be new, because the surface of the old aluminum pot has a thin layer of aluminum oxide. This layer of aluminum oxide prevents the aluminum inside from having a chance to "show up", and of course the "trick" cannot be changed.
Since the black substance is iron, it is not necessary to remove the black. If you want the aluminum pan to shine again, you can use vinegar to remove these blacks; cooking acidic foods such as tomatoes a few times will also dissolve the iron.
You can also put fresh apple peels into the pot, add an appropriate amount of water, boil for a quarter of an hour, and then rinse with water, the aluminum pot will become bright as new.