1) Priming with a black primer
2) Painting grey
3) Dry brushing a light grey
4) Washing with a black tinted wash.
As with the previous post on carving the tiles, I've pretty much stolen the painting process from the Black Magic Craft YouTube channel. The first picture shows the tiles after the black primer wash.
Like the primer, I mix up a large batch of the wash and keep it in a sealable container to use for other terrain projects. The plastic tub in the upper right is what I use to store the wash.