I just received a 2ft peace lily and the day after about 10 or more leaves turned black. I retuned it back for a new one. The new one now I have had for about three weeks. The Lilly bloomed but the inside stem is now back and looks like the flower is dying??? Thanks..

The black peace leaves could have been from over-watering or over-fertilized at the place it was purchased, or a possible fungus problem.
The flowers only last for about 10-15 days, so it could have been flowering for a little while before you got it.
Give your peace lily indirect bright light, they are a low to med light plant. Direct sun will burn the leaves to a crisp.
Water well when the top inch or two feels dry. They like semi most soil, not dry and not constantly soggy wet. Don't let the pot sit in excess water, or the roots will rot.

Once the flowers start to die, snap, or cut the whole stem off off at the base where they grow out from.