Getting rid of a Chalkboard Wall
I would give it  a good sanding with 100 grit paper and then 220 grit paper.
I would then give it two coats of a shellac based primer, it is very good at covering or hiding dark colors and stains that may otherwise bleed through other primers and paints. It is a very thin product and you may think that is not hiding the black board, but just because you can still see some black does not mean that it is not providing a sealed coating.
I would then give it coat of a good bonding primer, these are thicker and formulated to bond well to stubborn surfaces.
Now you have a nice primed surface that you can paint with any type of paint you wish.