cinder blocks to line a fire pit
Probably not toxic, but they are usually not a good choice. They do not tolerate exposure to fire very well. They do work as a second layer of protection.
I assume you are talking about fire pit in your back yard and not a fire enclosure inside your home. They are not a choice at all for inside your home.
Fire bricks, sheet iron or steel are the best choices for an outdoor fire pit. Cheaper cinder blocks and other concrete materials can be used as safety materials to keep people from coming in contact with the hot materials that are in contact with the fire, a second layer.
Penn State has some good safety advice.
Yes, it is an outdoor firepit. Thank you, Frank!