There are more genes involved in egg color. The blue egg gene is dominant over the white egg gene which colors the shell itself. Brown is more of a coating over white or blue shells that will make brown/tan and green/olive eggs. Some chickens have the brown gene, some don't. Looking at the inside of the shell, you can see the original color before the brown is added toward the end of the process. Don't know if brown has a counterpart gene, or just the absence of brown.
So by your example of a double brown roo and double blue hen, I think each chick will have brown over blue shells resulting in green eggs. Not an expert by any means, but I've been curious about this also and have read a ton!