I must admit I have bigger problems with finding blue key than with purple key, because purple one is visible from another place.
@egomassive, I think there are ghost markers, I see them in the level editor but not when I play the level. I think the water falling up is disguising it so that you can't see it or something.
It is because there are two almost identical screens (because of crossing corridors). In the game you can only enter the one without markers.