Look at the right back hoof for an example. It's almost crippling in that picture.
I noticed the right rear hoof right away. It looks like it’s stapled together.
Most good farriers will tell you that there is no biological or structural difference between pink hooves and black or striped ones. It's a myth that black hooves are harder. What's true is that a lot of Thoroughbreds have lousy feet. Some of this is due to the admixture of Northern Dancer's blood in the 1960s and subsequently, and some of it is due to the types of trims that are given to running Thoroughbreds, with a long toe and low heel. That gives more spring off the pastern and the possibility of more speed, but it also stresses the tendons and invites improper blood circulation through the hoof. Bad frog contact leads to bad circulation which leads to bad hoof quality.
Also, a lot of TBs have their hooves held together with Equilox, which is epoxy. You can imagine what that does to hoof quality over a period of time. Have you ever seen what happens to the nails of a woman who wears acrylic nails? Yes. Same disaster.
Add to those issues the fact that racehorses don't get good turnout on turf, they have their shoes constantly removed and changed and renailed, and you have a recipe for disaster.
That's a drastically abbreviated version of what's going on--it's much more complex and, to a non-horse person, boring. But I've seen quite a number of off-the-track Thoroughbreds who came home with horrible feet rehabbed with good farriery and trimming. It takes a year but they can get back to normal, or at least as normal as bad genes can make them.
Anyway, I'm not altogether sure that's a flaw in the hoof. Other photos don't show any darkness at that spot. Could be that's just mud and manure. Could be Equilox. Could be any number of things.
Not necessarily. I have an Appaloosa w/white feet who has the best hooves and is the soundest of my herd. I am able to keep him barefoot w/little or no hoof capsule distortion because I have a great farrier who keeps him well balanced! It may be that the racer just has manure or dirt packed up against the hoof wall making it look like it’s coming apart. The horses usually race in aluminum plates and it looks like there is a center clip on the rears and qtr clips on the front. Racing can be a brutal sport and there are a multitude of low level idiots with some extra money to spend that get involved thinking they can learn all they need to know on a You Tube video and that they have the next Secretariat. For the most part, serious owners, breeders and trainers give the best of care to their animals, they have HUGE dollars at stake and if they can bank some $$ behind a winner, they go on to be desireable studs and brood mares. I always say that horse trading is very much like used car salesmanship.....NOT for the feint-of-heart unless you KNOW what you are doing!! It is a sport in which most of your $$ gets flushed....VERY quickly!