I have two faces, but show you one.
I once had guests, but now have none.
I’m rarely bloody and seldom blue.
I’m often promised and sometimes new.
The moon. It has two sides, but on earth we only ever see one side. Man has been on the moon, but no one is there now. A full lunar eclipse is known as a blood moon, and the phrase “once in a blue moon” refers to something that rarely happens. Promising the moon is making an extravagant or impossible promise, and there’s a new moon once every lunar cycle.