It only reflects the available light. In your case, the blue light reflected is coming from the HPS lights themselves! That may sound odd but HPS does emit enough blue spectrum that you will be able to notice that the bags are indeed blue under the lights, but it should look like more of a navy blue since really there's not a lot of the blue light there to reflect to begin with. Considering that other grow bags are black, and reflect NO light, you're already ahead of the game.