yellow over blue

Yeah, the base coat is a great idea. I painted yellow over dark green (Judge Dread boots) and it several coats and was very frustrating.

I agree with Shinigami that you'll probably get the best results painting yellow over a white base coat. You can paint yellow over blue, but it'll probably take more coats to prevent the blue from showing through. Of course, it all depends on your paint hue and brand too, some yellows have better coverage than others.

You'll only get green if you're painting yellow on top of blue paint that is still wet, or, possibly, if you're using transparent/translucent yellow paint on top of a blue surface, which would allow the underlying hue to show through (much like transparent yellow over silver/chrome will give you gold).

From my experience with navy blue - don't even try. It's a pain the ass, not worth it. Paint the surface white and then yellow.