A blind corner is when one can not see what is around or behind a corner.
For example, when exiting an elevator, two blind corners automatically appear.
One should never approach or cross a blind corner while looking straight ahead. By the time your eyes pass the frame or edge, an attack (or most probably a simple collision) may be upon you.
Instead, look to the foot corners of the frame or edge of the wall that you are approaching. If a bad guy is waiting, his foot will always be visible way before your body ever crosses the threshold.
However, it will most likely be friends, family, coworkers, or just regular people that you will avoid colliding with. This is another way to use your martial skills to protect others and keep yourself alert.