On SAM: I'm good with it being either a Tank or DPS, but if it was DPS...that thing about them having a skill (passive? Never played XI) for doing more damage from the front has me thinking that they could still be a DPS, while being in the front. With OP's idea of countering maybe they could counter atleast the big hits like, say, Titan's Mountain Buster (because if it was a tank counter class it'd be doing it anyways), having it do a bit less damage to the tank and all that. Basically a tank's best friend that isn't a healer.

As for Blue Mage: This also interests me tanking wise. I hadn't really thought about it but since its whole shtick is about using enemy attacks...mayhaps rather than having it for keeps it could work where when an enemy uses a signiture attack on it they will be able to use it for a certain amount of time/the rest of the battle. Again think Mountain Buster...just bust it right back out on Titan himself and see how he likes it! Now of course how the Def would work for them is another matter entirely...