Moreover, why did my onions turn blue?

The pigments that give onions the colour behave like a litmus test. They are red in the naturally acidic onion. They turn green/blue in an alkaline environment. It sounds like when you cooked the beans it created that alkaline environment to cause the colour change.

Similarly, do red onions turn green cooked? Red onions—as well as other red produce, including cabbage and cherries—are rich in pigments called anthocyanins. When they’re cooked with acid, their color intensifies, but when combined with an alkaline component, they can turn a startling bluish-green color.

Also to know, is it safe to eat onions that have turned green?

Sprouting onions are safe to eat. Onions with a green spot are signs of decay.

How do you know when an onion goes bad?

Feel – whole onions should be smooth and firm. If they have soft areas it means they have begun to go bad. For a half, chopped, or sliced onions, they will have gone bad if they have a slimy texture to them. Smell – onions that are starting to rot will also start to smell bad.