The easy answer is because blue was the main color of the Whig Party in England - where the Tories were red.

Up until 1779 there was no distinguished color for the Continental Army - most soldiers in the army wore whatever they showed up with (and to be honest, speaking on the American Civil War - which I have done MUCH more research - volunteer soldiers still weren't always uniformed with blue or grey, but really what they brought with them (Hood's Texas army was known for their flannel)).

However in 1779 the Board of War passed a plan on uniform design for blue coats - their facings color was based on which colony they came from.

If this stuff interests you, take a look at the book Uniforms of the American Revolution by John Mollo... its a bit dated - but still really good stuff on this topic.