I have 3 new pre-seasoned cast iron pots as gifts I cleaned as instructed before using. But when I tried simmering a nice pot of beans in one the broth turned really black. I had to throw away everything now I am afraid to use them what can I do
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
It is best not to boil anything in a cast-iron pan or pot. The boiling water lifts the oil seasoning off the iron and exposes the raw metal again, leading to oxidation. Starchy foods, such as beans or potatoes, increase the reaction.