Well, if it is already textured I would start with some light grey as your base coat.
Then, water down some black gesso (acrylic craft undercoat stuff) and do a coat of that.
Some succesive coats will deepen then details (like what Silversixx said).
While it's drying I would gently dry brush some of the lighter grey onto the edges. Once this is dry I would go over it again.
I guess the temptation is do it in a couple of coats but if that's not working for you, the steps above could give you a different result.
That's how I painted the thing pictured here:
