Don't you notice Koishi when she enters your field of vision?
Her power is one that makes it so that she is visible in the physical phenomenon sense of photons bouncing off her, but people simply don't recognize what they are seeing is Koishi, so no.
Also, if you have blind spots in your vision, they aren't obvious. Your brain automatically "fills" the holes by blurring the things it can see over the holes. Hence, if you look at a white wall with a black mark on it, and move your eyes so that you have the mark into a blind spot in your vision, then you see a pristine white wall.
