Careful dry-brushing of black horses with dark blue produces subtle blue-grey highlights on the raised areas of the horses, picking out the musculature of the miniature animals as well as the strands of hair in the manes and tails.
Had a little time for dry-brushing those final nine horses last night. So, I dug out an old brush and got to work. Don't use your good brushes that still hold points for dry-brushing. Nothing kills a good brush faster! So, dip the tip of you bristles in the paint, brush most of the paint off onto a handy folded paper towel, and then quickly apply the remaining paint on the bristles to your figures. Dry-brushing is actually a much easier thing to do than it is to describe.
Anyway, here are a couple of photographs that will hopefully illustrate the benefits of this step for painting better looking black horses. Of course, you can skip this step if you like, but I prefer the extra subtle difference that careful dry-brushing with dark blue brings out. It seems to help make those heavy cavalry horses come to life.