This area you are referring to is known as the "optic disc". It is the area where the optic nerves and associated ganglion exit the back of the eye and connect to the brain. The reason it is 'blind" is there is a distinct lack of photoreceptive structures (rods and cones) available in this area. The brain does a nice job piecing together composite light images around the optic disk, but if you notice, when you hold your eyes straight ahead and change the positioning of your head, at some point objects become "invisible". They are still there, of course; you have quite simply adjusted your eyes to the point where the optic disk is receiving the incoming image. There are no receptors there, so reception is poor, at best.