It has always been possible to carefully adjust the two side mirrors and the rear view mirror so that using the three together, there is no blind spot. Most people are taught to adjust the side mirrors so that they can see the side of their own car--which is not really very useful. By rotating the side mirrors well beyond this point (so you can't see the side of the car), it is possible to eliminate the blind spot. With this adjustment, an object just moving off the right of the left side mirror will simultaneously be moving into the left side of the rear view mirror. Similarly when the object is moving off the right of the rear view mirror it is simultaneously moving onto the left side of the right side mirror. Thus one gets a continuous field of view to the rear using all three mirrors with no blind spot.
This is hard to explain--and somewhat hard to get accustomed to--but easy to experiment with. And you can draw some pictures to convince yourself that it works. The best way to test the adjustment is to drive down the center lane of a three-lane freeway and watch cars in the mirrors approaching from the rear. The approaching cars are always visible in at least one--and usually two--of the three mirrors.