I can't recall hearing the term outside of sport - a whitewash being a very one-sided game
I believe the sports usage means not merely any one-sided result, but specifically one in which the losing team is held completely scoreless.
http://www.bartleby.com/61/68/W0136800.html: whitewash: 3. A defeat in a game in which the loser scores no points.

It is evident how the basic definition (quoted below) gives rise to the defintion as "thin concealment", but unclear how it gives rise to the sports usage.

NOUN: 1. A mixture of lime and water, often that is used to whiten walls, fences, or other structures. TRANSITIVE VERB: 1. To paint or coat with or as if with whitewash.