How to clean my unanodized aluminium pot?
If it's a small spot you can rub half a lemon on it till it's gone. If the entire inside has darkened you can fill it with water, add several tablespoons of cream of tartar (2 per quart) and boil it for 10 minutes.

You can also buy aluminum cleaner, but the do it yourself is typically cheaper.

In case you're wondering why I'm suggesting an acid for cleaning. It's actually alkali foods that stained your pot, not acidic. e.g. boiling potatoes. Acid actually cleans the spots put there by alkali foods. 

The common warning against cooking acidic foods in an aluminum pot is to prevent excessive amounts of aluminum being leeched into your food.