Is it possible for me to use Westvale Abbey in a Commander deck that does not use black? I am making a Blue, Green, and White commander deck, and I would like to add Westvale Abbey. Could I use it?
No, you cannot. Westvale Abbey's color identity is Black.
903.4. The Commander variant uses color identity to determine what cards can be in a deck with a certain commander. The color identity of a card is the color or colors of any mana symbols in that card’s mana cost or rules text, plus any colors defined by its characteristic-defining abilities (see rule 604.3) or color indicator (see rule 204).
903.4c The back face of a double-faced card (see rule 711) is included when determining a card’s color identity. This is an exception to rule 711.4a.
And the definition of color indicator:
204.1. The color indicator is printed to the left of the type line directly below the illustration. It consists of a circular symbol filled in with one or more colors. A color indicator is usually found on nonland cards without a mana cost.
204.2. An object with a color indicator is each color denoted by that color indicator.