What sort of primer should I use to prep a marble fireplace?

Ok I know it's an odd question but I'm a childminder and have recently moved house, the room which I have bagged as my playroom had a fireplace in with a "marble" fireback. Originally I tried to stick a piece of painted hardboard against it as my blackboard, but this wasn't successful.

I now want to prime and paint the marble itself to use as my blackboard, but aren't sure what sort of primer I need to use, so am looking for the answer here please.
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marble fireplace