Merle is one those things.
The color itself is inherited through an incompletely dominant allele, which means you can easily transmit it from breed to breed. All you have to do is cross a merle dog of one breed with a breed that doesn’t yet have the color, and then you just breed back into that second breed until the dogs all look like like that breed, just with the merle color.
The problem with the color is that it can be risky. It is often frowned upon to breed merle to merle, because there is risk that some of the puppies that are born double merle can have closed ear canals or eye deformities that might be as severe as the dog having no eyes at all.
All the breeds that have a standard merle color have a way of dealing with this problem, though, to be fair, not all breeders follow those guidelines.
The problem happens when you have cryptic merles– dogs that have the merle variant but you can’t tell by looking at them. The dog simply doesn’t show the pattern at all, and when someone breeds that merle to a known merle, you can get the defective puppies.
Also merle can be hidden in yellow or cream colored dogs, simply because they don’t produce any pigment on their fur but yellow or cream. French bulldogs come in this color. (Though I don’t think their fancier use this name).
So you could run a risk in creating double merles entirely upon accident.
Now the dog in the photo doesn’t look to be that far removed from the crossbreeding that introduced the merle variant into that particular strain of French bulldog. But there are breeders who are producing merle French bulldogs with much more typical conformation.
I don’t see the point in adding merle to a breed that also comes in cream or yellow. It’s way too risky.
I’m sure the French bulldog clubs are thrilled to know that this color is being produced. It’s not a standard color in the breed.
But the issue is managing merle in a breed population, and I guarantee that the people who started producing these dogs aren’t thinking about breed management issues.
People love that merle color, but if it’s going to exist in any breed, there have to be guidelines about how to produce it.
Otherwise, it’s just asking for misery.
It’s better off left out of most breeds.