Parents with green and blue eyes, respectively, with brown- and blue-eyed children?! Is this possible?

Green eyes are governed by a more complex process than just the genetics you've been learning about. There are a bunch of other genes that interact with each other to make green eyes. So rather than it being the straight Bb or bb genotypes, there are other genes that can be ped down to interact further and make green eyes (or hazel, or any other odd colour!). So it's possible that your grandfather has the genotype Bb, but also has other genes that were not ped down to all his children. Hope that makes sense!