Although not common, these color deviations do appear, if conditions are right in the wild they will survive. Just recently I have seen pics of an Herichthys minckleyi in the wild, with the same coloration.
And in spawns of fish in my tanks (although not this drastic) I have seen the similar mutations. Out of a spawn of @800 managuense, I found 3-4 almost black jags, and 3-4 without any spotting at all.
I believe this is natures ways of protecting the species, is geographic conditions change. If a sudden earth quake or flood leaves the normal colored species vulnerable, this may allow the color mutation to take over and basically save the species from extinction.