Several of the sources I've read say "up to a quarter were Black
or Mexican..." which I always took to mean 20% Mexican, 5% Black.
It's pretty much a "fudge factor" for historians who want to be PC but can't fudge the numbers to their satisfaction any other way.
As an aside, a family member of mine from Clayton, New Mexico told a story related to her by her father from the turn-of-the-century about a group of cowboys coming into the "Big" hotel in town (still standing and still serving dinner last time I was there, the Eklund Hotel). When they sat down, the waiter said "You boys can eat in here, but your nigger has to eat in the kitchen"
In reply, one of the cowboys drew his big Colt and laid it on the table and said "Nigger Charlie eats with us at home, Nigger Charlie eats with us here." End of story.
As far as Mexican Cowboys goes, there were surely plenty. There are always stories of "So and so shot a Mexican, etc..." or "I got into a gunfight with a Vaquero" in the ruminations of such lights as John Wesley Hardin, etc., who in fact got into a gunfight with the Mexican boss of the herd behind him when driving cattle to Kansas. But that's like saying that there were always Germans around in the West. There were, but that doesn't make them a huge percentage of the population.
Anyway, the moral is that, indeed there were both Blacks and Mexicans working cattle in the West (after all, Mexicans invented the technology that the American Cowboys adopted), but as to their numbers? Good question, but I doubt that there were 25% of them Black.
Cheers,
Gordon