Horse colors are due to the interaction of a large number of genes, many of which we’ve discussed. These may be divided into base color genes, diluting genes, genes that cause interspersed white hairs and marking genes. One type we have not discussed, because the genetics are not really understood, is interspersed black hairs.
I’d like to point out one thing that Sponenberg does not cover: there are two distinct types of darkening due to the presence of black. It takes a magnifying glass and a great deal of patience to tell the difference, but darkening can be due either to interspersed black hairs (called sooty and it can occur on any base color) or to hairs that are red/yellow at the base and black at the tip (producing a shaded appearance and I think occurring only on bay, wild bay and some seal brown horses.)
For the rest of this discussion I will assume the horse is of one of the base colors, but sooty and shading can occur with any dilution or marking genes, or together with roaning or grey. You just have to remember what the other genes do to red and black pigment, or if they have different effects on coarse and fine hair.
Liver chestnuts are often sooty. It takes careful examination to tell if a chestnut has interspersed black hair, but when I was examining them with a magnifying glass, this was true of every liver chestnut I examined. Even red chestnuts often have a few black hairs mixed into the coat and the mane and tail. Bays can also be sooty, but this may be confused by the presence of shading.
In order to understand black shading, it is necessary to go back and look at how the agouti locus affects mammals in general. The locus got its name from a middle and South American rodent, the agouti. This animal has fur in which the individual hairs are banded in black and yellow. As it happens this is very common in mammals, and a number have banded hair. Unless the hair is very coarse this is not obvious—wild gray mice and rabbits, for instance, really have hair banded in black and light yellow.
The banding may vary from multiple bands on a hair to hair with red/yellow/cream bases and black tips. The banding may also vary with type of hair, with some hairs (often the coarsest) being solid black and others (often the finest) being predominantly yellow.
Remember bay and wild bay are due to genes at the agouti locus. Most bay horses have at least some banded hair on the body, usually with a red base and black tip. This is easiest to see around the edges of the ears, and the banded hairs tend to be most numerous along the spine and spreading down to cover the hips, shoulders and upper barrel. It’s been a long time since I actually looked at individual hairs with a magnifying glass or under a microscope—I was doing this in the late 60’s and early 70’s. But as I recall, just about every bay or buckskin horse I looked at had at least a few black-tipped hairs. In some, the black tipping produced a shaded effect on the body of the horse.
A few horses look black or seal brown in summer coat and quite different in winter. I recall two of these. Duchess was a typical seal brown in the summer—black with tan shading on her flanks, muzzle and under her tail. In winter she looked dark bay. Careful examination of her winter coat showed red near the skin with deep black tips. In her short summer coat, apparently only the tips showed.
The other was even more striking. I first saw him in winter coat, and thought at first he was a blue roan. Careful examination of his coat showed yellow bases with black tips rather than interspersed white hair—a buckskin with deep black tipping. In summer, I knew he was the same horse only because the stable owner identified him—he was a typical seal brown.
This is an area that needs much more research. Unfortunately with the prominence of DNA in genetic research, researchers seem not to be paying as much attention to the distribution of pigment in the hair.