In case you want to try this yourself, here's the process I use.
I start with a fine art paper; the one I use is Zerkall Frankfurt. I cut the paper to size and tape all the edges to a firm board. I take a very watery watercolor wash of Ultramarine Blue (or Ultramarine Blue mixed with Burnt Sienna for a darker wash) and I cover the entire surface.
When the paper is saturated with water, it will buckle and bubble, pushing the paint to congregate in the "valleys".
I leave the paper taped to the board while it is drying, and, strangely, the paper flattens out nicely, leaving the paint pigment where it had gathered and creating interesting textures and forms for the background.
For the drawing, I use Stabilo pastel pencils in various hues of Red and Sanguine and white charcoal for the highlights. At the very end, I use a Prismacolor pencil in Terra Cotta for my darkest values. The pencil is waxy and the change in texture adds emphasis to the darkest areas.