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. It's not rare to see two full moons in a month. That's every thirty-three months on average. Months have different lengths, so the phenomenon moves around a bit. If you think about it, it's a little like getting paid every second Friday and finding some months you get paid three times instead of twice.
Meaning is a slipperly substance.
I have counted six different meanings which have been carried by the term, and at least four of them are still current today. So that makes discussion of the term a little complicated. Both were used as examples of obvious absurdities about which there could be no argument. 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. In 1927 a late monsoon in India set up conditions for a blue moon.