Hello,
I've heard of people mentioning it before, here on kingsnake.com and elsewere on the web. The only caresheet that I could find that mentions a link between temperature and the rate of the ontogenetic color change is the one at centralpets.com... the link is below. Several other credible sources (ks.com caresheet, VPI caresheet, Vivarium article by Alan Kardon) don't say anything about it. I was looking around at archived versions of Critter Connection's gaigeae web page, and their original CB '96 breeders from Mark Bell and San Antonio Zoo lines took 3-4 years to turn completely black. They also say that they were keeping them relatively warm for those first few years, because they were in the 80-85 degree reptile room with all the other snakes. It wasn't until a couple years later that they started keeping the gaigeae at cooler temps, and the ones they raised later on seemed to turn black in a more reasonable amount of time.
Nate
Black Milk Caresheet