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.
What am I?
You are:
THE MOON
I have two faces, but show you one.
The Moon has two faces in the sense that from our perspective it has a "dark side" or "far side" (which we never see) and the side which faces us - the oddity is that we have this relationship with the Moon.
I once had guests, but now have none.
This refers to the astronauts which have visited the Moon - there are currently no humans on the Moon - nor have there been for quite a while.
I'm rarely bloody and seldom blue.
This refers to the red and blue moons. These are rare phenomena c.f. "once in a blue moon"
I'm often promised and sometimes new.
The "new" moon is about the lunar cycles. Finally, one often "promises the moon" i.e. makes crazy promises.
Ideas:
Why not something about honey :D
Accepted answer be damned, you are a
delicious, delicious steak
I have two faces, but show you one.
If you get a delicious, deliciou steak it will red/pink/brown on top, and brown/black on bottom, depending on how well it was done.
I once had guests, but now have none.
Farmhands have to check up on the cows very often before they are converted into delicious, delicious steak.
I'm rarely bloody and seldom blue.
Rare, bloody, and blue are the three least-well done rarities of delicious, delicious steak. (Well done, medium well, medium, medium rare, rare, bloody, and blue. Sometimes "still mooing" makes the list, but only as a joke.)
I'm often promised
A steak dinner is generally considered the most elegant dinner available to most people.
and sometimes new.
Veal, the meat of calfs, is considered a delicacy because it is so tender.
My guess was
teeth
I have two faces, but show you one.
Only the front of your teeth is usually visible.
I once had guests, but now have none.
Teeth start out with temporary "guests", baby teeth, next to them, until they fall out.
I'm rarely bloody and seldom blue.
If you lose a tooth, which is (hopefully) rare, your mouth becomes bloody, and your teeth are usually only blue when you brush them with blue toothpaste.
I'm often promised and sometimes new.
This is where it breaks down, although you can "promise" your teeth in a half-smile, as well as getting new teeth if need be.
I would have said:
A quarter
I have two faces, but show you one.
Heads or tails, only one is ever showing
I once had guests, but now have none.
There used to be dead presidents on US quarters, and were replaced by representations of individual states.
I'm rarely bloody and seldom blue.
This line fits the least. Could be a reference to blood money (rare) and sometimes coins have a blue tinge to them
I'm often promised and sometimes new.
The check is in the mail! :) And freshly minted coins
A pregnant lady giving birth has two faces: her own face and the baby's. She had a guest in her body, but now it's out. She isn't bloody because she can't menstruate while pregnant and she isn't blue because childbirth is a happy thing. Having a baby is often promised or agreed on and the first time giving birth is a new experience.