Black stone plant pots gone white - what can I do to put back to black?

I spent a lot of money on 4 square large plant pots - they are black stone, sort of granite effect. When I purchased them they were beautiful and black. Two winters later they are going white on the outside. I can't take them back as I have tried varnishing them to bring back the blackness! Anyone else had this, or can you recommend something to try to bring them back to black?? Thanks