It's also highly radioactive. (This is the stuff they used to make the now banned "black boards" out of).
Think you are confusing slate (the original blackboard) with transite blackboards. The original Transite was asbestos reinforced cement board and was used in place of slate for a lot of applications such as blackboards, lab countertops, roofing tiles, etc. There were a lot of other applications for Transite (fire-proofing, electrical insulation...) Ended up on the poopy list because of asbestos. (Transite 1000 is a non-asbestos product.)
They didn't exactly glow green in the dark but they'd set off a geiger counter!