I wouldn't suggest a dark knight subjob, but whatever floats your boat.

As far as WAR or MNK, you'll end up leveling both. WAR is a good sub job for both SAM and DRK, so you'll want to have it for when you're leveling those. MNK is the basic sub job for WAR early on, so you'll have to level that as a sub for WAR at some point. On top of that, I'd suggest leveling THF as well, as THF is a sought-after sub for both SAM and DRK after a certain point.

Anyway, I'd lay off the DRK. About all it offers to a SAM is the attack bonus. Even as a tarutaru, the DRK sub's MP pool will be pretty pitiful, and the halved magic skills won't be strong enough to warrant use of spell casting. Sure, you get more STR, but a WAR takes care of that just fine while adding more useful job abilities. THF is also good for sneak attack/trick attack (SATA) past level 60, and even just SA past 30.