The wash can go at several different points in your rock-making process.
Early on, it will alert you to the details (or lack of details) of your rocks, so you can be satisfied or carve more.
After applying your base coat, it will both alert you to details and start emphasizing negative areas and shadows.
You can also produce details to the surfaces of your rocks by dry brushing, which, if done properly, will form texture, or you can use the tip of a still, wide brush to stipple detail onto your rocks if your paint is thick enough and fast-drying enough. Ultra-flat latex wall paint is pretty good for this.
Once again, a wash (doesn't have to be black, but should be a darker color) over the DRIED stipple or dry-brushed texture will bring it out.
For a different texture, you can add sand, or irregular dirt to your thick latex or acrylic paint. Bring it out by both washing and dry-brushing. This works really well for sandstone, or conglomerate cliffs or rocks.
Have fun! Rock carving and scenery making is the cheapest, fastest way to add to the overall quality of your layout! Most bang for your time and buck.
PS For "realistic" rocks and colors, reference photos are an absolute must. If you carve rock while looking at reference photos, even though your carving may not look exactly like what you see, the realistic quality will increase exponentially over "imagineered" rock carvings.
