Can I paint the inside of a fireplace and with what?
I have recommended to people on this board that they abandon re-painting the fireplace and instead construct a small panelboard insert that you can put into the fireplace to cover the ugly cinderblock for candles, and remove when you want to use the fireplace for a fire. The cinderblock backdrop is there only for fireproofing, not decor: why keep it for candles or when the fireplace cannot be used for fires? Creating a panelboard insert gives you something better than a cinderblock backdrop, while not creating a hazard in the future.
Three simple panels connected with hinges so that it folds is all you need. Home Depot or L-O-W-E-S or other building supply centers carry the pressboard/panelboard that you can keep the original color or paint with regular paint to your heart's content.
I have seen many painted black with heat resistant paint from the auto parts store. Any that gets burned off will end up blackened anyway so the whole thing is black on black. Any noxious fumes go up the chimney.