Black horses are the result of a recessive color caused by two gene combinations. To be black, a black horse must have at least one copy of the dominant gene for black melanin (known as big "E"). He must also have two copies of the recessive non-agouti gene (known as little “a”). The presence of at least 1 agouti gene causes the black coloration to be restricted to the mane, tail, and body points resulting in a bay horse. Therefore, to be black you don’t want the agouti gene (known as big "A") to be present.
Within the color black, there are various shades. The sooty modifier adds black hairs throughout the body. This modifier is responsible for making bay horses “look” like they are black. A black horse with the sooty modifier may appear blacker throughout.
The silver modifier (not present in the Spanish horse breed as far as we know), lightens the mane and tail and body – resulting in a brown chocolate horse with flaxen mane and tail. There are other modifiers as well. The crème gene (dilution) added to black results in a smokey black – although it is difficult to distinguish a smokey black from a true black since the dilution does not really affect black hairs. The roan gene turns a black horse into a blue roan. The dun gene makes a black horse a grullo. The mealy effect gene causes a lighter shade on the muzzle, flank and inner thigh and results in a seal brown horse (actually a type of black). And of course, the grey modifier gene turns a black horse to grey or white.
Most black horses will experience fading over the summer season. Fading black horses get reddish-tinged hairs or brown burned areas from sweat or sun. They may also fade due to poor nutrition. To bring out their best color, fading blacks must be kept out of the sun or blanketed so that their hairs are not bleached.
Fading black horses are still blacks, even though during certain times they may look dark brown. Black horses are born a smoky color, or sometimes dark bay or brown.
Non-fading black is a blue-black shade that does not fade in the sun. It is called blue black, raven black, coal black, or jet black. Genetically, this horse cannot be differentiated from a fading black horse. Non-fading or jet black horses are common in the Friesian and Arabian horse breed.
Fading black horses are a common form of black in the Andalusian horse breed. Bay blacks (or bay horses with the sooty modifier) are also common. Jet black Andalusians are more rare.