What is the origin of the phrase 'blue moon?

How many songs reference the phrase "Big brass bed", and what's the origin of the phrase?

• I've heard it so far in Dylan's "Lay lady lay", Young's "Out on the weekend", and in what I think is the origin, a song performed by Blind Willie McTell (can't remember the name right now, it's sung by a woman in the recording I have.) what's the story behind this phrase?

• Answer:

  The song you're referring to is Rough Alley Blues by Blind Willie McTell and Ruth Mary Willis which refers to a Big Brass Bed. Dylan was definitely aware of Blind Willie McTell (he wrote a song about him after all) and this is no doubt where Bob got the phrase from. As to whether this is the origin of the phrase it's probably impossible to know. McTell was part of a longstanding blues tradition that borrowed phrases, lines and songs from other tunes. People weren't worried about copyright because nobody was making money from their music they were just playing music in clubs for anyone who would listen. McTell may have come up with the phrase on his own or he might have heard it in a song by a guy who heard it in a song buy a guy who heard it in a song... How many other songs reference it? I've got no idea. Those are the only three that I'm aware of but there could be dozens more.