We had a black piece of plywood over our firebox when we moved into our 1850's home 12 years ago, and it is still there! So I decided to decorate it a bit. Until the chimney/fireplace repair money is saved up! Chalk will work for now.
Our 1850's home has four fireplaces. One fireplace was covered in 1970's square bathroom tile. And it was ugly. We removed the tile that we could (which was NOT easy!) and then covered the existing tile and base with mortar.