Haha, I have a mutant... er X-horse as well (avatar pic). She basically looks black save the light brown points on her flank, nose, inner legs, etc.
Getting back to dapples, I've heard the same thing about it being a sign of good nutrition and internal health, but wasn't sure if that was just an old wives' tale or not because I find it difficult to find any reliable articles on the topic. Some things I've read suggest they are genetic and will show up when the horse is in good condition - which would explain why not all healthy horses get dapples. My mare dapples year round, but in winter, they are not as obvious - it kind of looks like she lay on a chain link fence, for lack of a better way of explaining it.
I'd be really curious to know the exact cause of them though. For example, if I saw a dapple grey horse, I wouldn't necessarily attribute its dapples to health, but more to just strictly pigmentation, however, maybe I'm wrong on that? Maybe a grey horse won't dapple if it's not healthy? I'm not sure I've seen an unhealthy dapple grey horse to be able to properly tell.