Originally Posted by SSP
With a glasses-mounted mirror like my Take-a-Look, there's really no "blind spot". Unlike the fixed mirrors in cars, a helmet or glasses-mounted mirror can easily be used to "sweep" the entire area behind you. The only true blind spot is right next to me, and a simple glance to the side (not requiring a "look back") is all that's required to assure it's clear.
IMO, the mistake the mirror-wearer in your course made was in not signalling his intentions.
I use a Take-a-Look and I would never move laterally without a look back first, even if the road appeared empty in my mirror. If nothing else, I just think it's a good habit to instill in oneself.