what made the juice/water of the beans turned black
It sounds you ended up with magnetite in your broth. That is the black stuff you want to use to season cast iron pans by binding it with fats. Sounds like that went wrong with your pans. If you really want to use them for boiling, you need to season them again, and test if that was successful...(water with a drop of lemon juice or two or three as a test). It is not poisonous.
If you don't know how to season a pan, this is the method I like to use: guide for seasoning a pan