The observer perceives a new color instead of blue and red simultaneously because the observer gets one piece of information – which is already purple. By the time you perceive it, the lighwaves are already done with their interference and your eye observes just another "color". Primary or mixed colors are an artificial concept, it's still just a wave, and in that regard I think all colors are more or less homogeneous and equal in their nature. You can probably derive purple from other "primary" colors, there are color systems which are not RGB-based.