According to common folklore, based on a mistake printed in Sky and Telescope magazine in 1943, the blue moon occurs when there are two full moons in a month. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it's not often two fall in one month.
So a 'blue moon' has nothing to do with the color -- most of the time.
According to NASA.gov, "Squeezing a second full Moon into a calendar month doesn't change the physical properties of the Moon itself, so its color remains the same... However, be aware on rare occasions it can happen."
What does it take to get an actual blue, as in the color blue, moon? "A truly-blue moon requires a volcanic eruption," according to NASA. That's when smoke from a volcanic eruption here on earth rises up, making the moon look hazy and, well, blue.
"Certain forest fires can do the same trick," according to NASA.
Those truly blue moons are even rarer than the other, already rare, blue moon. The next of which will occur on July 31, 2015.
Friday's blue moon reached its fullest point when it wasn't viewable in Chicago-- at 8:59 a.m. But don't worry; you can get a glimpse of the rare event tonight, starting at 7:09.
Blue moons occur approximately every 2 and a half years.