GTPs are of certain locales have black scales. Signal Herp has produced some really awesome high black snakes. Perhaps she just has some black in her?
Typically when a snake has black scales because of humidity they are on the belly and lower sides of the snake.
Imho, if they are random on the top of the snake they are probably just part of the snakes pattern. Also, the tail of the snake is usually a darker color as well. This is because they use it for caudal luring.