If you need a telescope, see Harry. The Harvest Moon is the best known of these, lighting up the fields sometime after the fall equinox, enabling work to continue late into the night. Blue Moon is different from the monthly or seasonal moon names as it isn't restricted to a time of year. It is a movable feast. The older meaning may be wishy-washy and the newer one solid and technical, but don't let anyone tell you they have replaced one with the other.
Meaning is a slippery substance.
I have counted six different meanings which have been carried by the term, and at least four of them are still current today. That makes discussion of the term a little complicated. Both phrases were used as examples of obvious absurdities about which there could be no argument. Or that it would be on the Twelfth of Never. When the Indonesian volcano Krakatoa exploded in 1883, its dust turned sunsets green and the moon blue all around the world for the best part of two years.