Answer:
You can see nothing else, When you look in my face, I will look you in the eye, And I will never lie.
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.
It has no top or bottom, but it can hold flesh, bones, and blood all at the same time.
What looks back but cannot see?
What's black when you get it, red when you use it, and white when you're all through with it?
My days are numbered. What am I?