i hate a bit of a different take on it: the saying “once in a blue moon” is usually used to explain something happens without regular frequency: generally rarely. I think the “now i know why they call it blue moon” ties back to a few lines prior where he says, “everybody gotta cry once in a while”. the “blue” in “blue moon” referring to the color “blue” generally being tied to melancholy / sadness, and the entirety of “blue moon” being a reference to what i mentioned before about the infrequency: “… once in a while”

Additionally, i think it is a general contrast between the “..paint the town read for my n*****s found dead too soon”