Chestnut is a base color, Chestnut is also recessive to Black, so should a horse carry both a Black, and a Chestnut (red) gene, then the horse will automatically be Black because Black is dominant. A Chestnut horse is always known as (ee). That is simply the genetic make-up of a Chestnut horse. Now granted, a chestnut horse can carry the Agouti (A) gene, but this gene will be hidden as it only shows up on Black based horses, not a Red based horse.
Chestnut has been in the Percheron breed probably throughout the entire existence of the breed. Even breeds such as the Friesian who is bred to be strictly Black, has not eradicated the Recessive Chestnut gene. However, now with the simple dna hair testing that can be done, Chestnut could be bred out of the Friesian breed. I would not recommend it though however as it lowers your gene pool even more. I also would not recommend this being done in our breed as it would also lower our gene pool to breed from. A quality individual should never be discriminated against based solely on color if that individual is quality in all other ways that truly matter more. In the case here in America, Chestnut is an accepted color, so the discrimination is only a personal belief. There truly are a handful of good Chestnut mares out there who in many cases have better characteristics than their Black breed mates. When these red mares are bred to a homozygous black stallion the resulting foal Will Be Black. Granted this foal will also carry a chestnut gene, and could produce chestnut, but should it always be bred to a homozygous black horse you would continue to have nothing but Black foals. Thus you wouldn't eliminate what could possibly be an entire genetic line or group of lines while maintaining a more desirable color.
The last known large breeder of Chestnut Percherons was Cedar Grove Stock Farm of Washington. Bob Watson has been breeding for Chestnuts probably more so than any other person.
Best known lines for Chestnut Percherons and the recessive gene is; Highview Dragano, M.G's Prince, Blue Ribbon Farms Hercules, A large number of Justamere Showtime horses, and Dragano son's and daughters. Even South Valley Did-It himself was a red carrier. They are out there, and a number of black Percherons do carry the chestnut gene. All is not lost if your horse produces a chestnut foal for you, I know a handful of people who are looking for exactly that. :)