He didn't really eat it so much as chew it up into tiny pieces and spit it out. He managed to get blue ink, washable and non-toxic for kids luckily, all over the carpet plus bits of chewed up blue marker plastic. He ended up with a blue mouth and tongue too.
I guess the lesson here is not only to not leave blue markers lying around, but to make sure to pay a bit of attention to my dog when friends come to visit. At any rate, I got a good laugh out of it. Who could really be mad afterall?