Eregion has a good amount of Black Ash and on my server I never seen anyone farming it. Hell, I hardly ever see anyone in Eregion at all.
Another place I like to go is the valley leading up to the Bitter Stairs. If you stick to the sides of the valley you should do decent. It's not super dense, but there is a decent amount to get and will always respawn before you get back. I think the reason I like that area is because I always go there with my explorer and end up with a good bit of dwarf iron and platinum as well.