Can I use Extort in a deck that isn't both white & black?
Yes, because extort is merely a keyword. The color symbol {hybrid white/black} is not part of the card, but only exists in reminder text, which is not officially part of the card.
Here's the official ruling on the matter:

EXTORT
With the release of Gatecrash, we'd like to clarify our stance on extort by confirming that, as always, reminder text has no effect on color identity. Extort adds no colored mana symbols to a card outside of its reminder text, and therefore is legal for play in Commander decks of any color or colors (as long as the card it appears on is otherwise legal, of course). There will be no updates to the rules in the foreseeable future that affect how extort works in Commander.