As Soviet Russia declared communism wouldn't work unless ALL countries of the world turned communist, liberals believe their principles can work only if they have a monopoly on all thought.
If the Washington Post is indeed concerned with balance, and not a monopoly of liberal thought, it will hire another conservative blogger-- one that liberal bloggers despise.
What's more likely to happen, though, is a "maverick" conservative along the lines of John McCain or Andrew Sullivan (who blogs for Time.com) will be chosen, as Post editors strive to abide by the rules of liberal orthodoxy while appearing balanced.