There is a blue moon tonight apparently, although there is some dispute about what the exact definition of a blue moon is. A blue moon is not blue. Those sources that talk about there being a blue moon tonight define blue moon as the second full moon within the same calendar month. Other sources say that this is not the correct definition, and that a blue moon is actually the third full moon in a season with four full moons.
Why blue? It probably comes from the Old English word belewe, meaning 'betrayer', so, a 'false moon', given this name by the clergy since it was a moon that appeared in the spring, but that was not the correct moon for determining the date of Easter.
The OED is no help as to what the original meaning of 'blue moon' was, or how the expression came about. It only gives the colloquial definition 'a rarely recurring period'. However, it does give some useful information about the word blue in general. The spelling blue only became common, thanks to the French influence, after 1700, and the pronunciation 'bl-you' (phonetically /bljuː/) was common until the late 19th century. Middle English spelling was blew, and there was an Old English word bláw. The OED says that the Germanic blæwo- may be a cognate of the Latin flāvus, which means 'yellow'. It seems odd that blue and yellow should be connected linguistically, but the OED reminds us that the names of colours have undergone changes over history. As I said in a recent post, Homer never used the word 'blue' but instead described the sea as 'wine-dark' or 'wine-looking'.
Here's a Guardian comment piece about today's blue moon. As the author says, although 'once in a blue moon' means 'very rarely', in fact, a blue moon is not that rare. He says it's not as rare as a Preston Guild, which I wrote about last year (here).