I am also afraid the literature on gekkonid colour change is thin at best. They may have numerous factors affecting colour change. Although your answer is extremely thourough, the fact of the matter is there CAN theoretically be a blue crestie. The variation in purple to lavendar hues in such geckos is at the 'low' end of the blue spectrum and let us not forget those remarkable day geckos with TRUE blue tails and spots. I'd have to agree with you on one point though, you probably not find a true blue crested, but with time I am sure there with be some pale 'blueish' types ready to gauge the market.