Thermal vision isn't meant to be blue, it depends on what colours the mask uses to interpret the differences of heat. Which are signaled by infrared light.
But what i mean predess, is humans can see the vision as blue, yellow, red and all the other colours because human eyes can detectall colours of "visible" light, while preds only see the infra red light. Therefore the red thermal vision would make more sence than the very colourful one, because their eyes would only see as black anything that gets too far from a shade of red, since the predator saw only black and red in the movie. And their masks would have to transmit infrared light to their eyes, which means humans couldn't see anything through it, because they don't see infrared.