Do you have photos of the adult melanistic pair that we could take a look at, and do you know their age and if they are a proven pair?
Few breeders are working with the melanistics in favor of the extreme vivid colored thayeri available today, which is a real shame in my opinion. People looking to buy variable kings usually opt for the flashy ones which explains their scarcity in breeding populations. That said, you shouldn't expect to be able to sell melanistic babies for top dollar. I myself am looking for a nice male and possibly another top notch female, I plan to work with the melanistics, just to preserve them in captive populations as I feel they are an important part of what thayeri are all about.
Early this past spring I was able to purchase what you could consider one of the best melanistic females money can buy, she is a big, healthy and beautiful jet black(her chin, throat and entire ventral is completely jet black with not a hint of color anywhere) '02 sub-adult. She cost me $75. I bought a pair of females actually, the second was an '01 model and was a proven breeder... not quite as nice and totally jet black as the '02 but very close, I paid $60 for her. But... I feel I got a really good deal. I myself would be comfortable paying $100 for top of the line sub-adult melanistic animals. I recently spotted a nice '04 male melanistic hatchling in a breeders rack for $45. That should give you some idea as to what people expect to pay for the melanistics.
I also have a couple dozen young high-end future breeders with all the intense color and pattern variations from the top breeders in the country, these are the ones that draw the big dollar from collectors. I will not be breeding for profit as my main objective however, I will be doing it primarily for the excitement and knoweledge. But see there, even I lean towards the outrageous colors, and I paid a freakin' small fortune for my thayeri collection!!! Anywhere from $100 to $300 each, the exceptions being the melanistic females.
I wish I could give you an answer to your question on the genetics of the melanistic trait but I can't...
Mike