If you want to really put a tinfoil cap on, you could say that by Google contributing to WebKit they are giving Apple a lot of free code, allowing them to devote fewer resources to their browser. Once Blink diverges father from WebKit it won't be practical for WebKit to merge in changes from there.

One has to wonder if Google will be recruiting other WebKit contributors (RIM, Intel, Nokia, etc) to move over to Blink. This would put Apple in a tough spot.