The color black appears when a horse has a black gene (is not solid red) and has no bay (agouti) genes.   Since bay is so common in the equine gene pool, the black color is proportionately rare.

Lee Mars Lady AQHA mare "Lady" and her colt. Owned by Becky DeMuth. The foal is "Lee" - registered name of Ebonys Lucky Miracle pending. (He was born without a heart rate or breathing!) He is by The Ebony Impression (also black).

The Ebony Impression, sire of colt above. AQHA and NFQHA registered.

Tuxedo, owned by Milynda Milam  of Red Fox Farm, is a black Thoroughbred stallion and is tested homozygous for black. Lower picture is of a Tuxedo foal born this year, a black TB colt, "Black Tie", at two days old.  Black Tie is also homozygous black, since his dam also tested homozygous black.

Ever the Lady, 1993 black QH filly, a few hours after birth....as you can see the dam is a seal brown. Most black horses are not born jet black (a pewter-grey color is more common), but some are!  Submitted by Julia Lord

"PK Ebony Bonanza"- age 14 months Owned by Pamela House Sire: Ole Sir Brian, Bay.  Dam: Elite Bonanza AQHA Dunalino (Red Dun) by Blue Chip Bonanza a Grullo son of Coys Bonanza.

A quick word on the color "black". 

There are two known pigments in the horse's hair: red and black.

"Solid" black is a rare color in the equine world because there are so many genes that modify it into "something else". 

Spots: we're not going to address white spotting on this site at this time.

The most common modifier is the bay gene, which is present in most horses alive.  The bay gene confines the black color to the mane, tail and legs of the animal (its points). When a red horse -- chestnut, sorrel, palomino, cremello --- has a bay gene, it is invisible, because there is no black to be expressed on the points.

A black horse with a cream gene is a smoky black, which may be mistaken for "brown", "dark bay", or "seal brown".

A black horse with two cream genes is called a smoky cream, and believed to look like a perlino, but as of this writing no confirmed examples of the smoky cream color are known to this author.

Another gene that can influence black color is the dun gene.  When the dun gene is present in an otherwise black horse, it becomes a grullo, with a more or less mousy grey body with black points, a dorsal stripe and leg barring.  We are not going to go any further into this gene, on this site, at this time.

But when the black pigment is present in a horse with no cream genes, no dun genes, and no bay genes, finally you will see a solid black horse.  Even this color sometimes will bleach in the sun, etc. and show tinges of brown, red or orange.  But when the new coat comes in each season, it will come in looking "true black" all over.