What does the term blue money mean?

"Blue Money" is a song written and recorded by Van Morrison in 1970. What Blue Money refers to in the song, and in most uses of the term, is money earned from salacious or racy photographs and images.

So, in the Van Morrison song, when he sings "The photographer smiles, take a break for while, do your very best... when this is all over, we'll be in clover, and we'll go out and spend all of your Blue Money," he's watching his woman have naked or nearly naked pictures taken of her and is looking forward to spending what she makes- the aforementioned Blue Money!