I've got a dark/katana tank.
Katana is a really fun set, and dark armor brings a lot of different tricks.
As for the meta thing, I wouldn't sweat about it too much on a tanker. The defense meta is partly because it's easier on non-tank ATs to cap defense and s also partly because non-tanks/brutes have a lower resistance cap.
Rad armor brings zero defense to the table but is still considered top tier. In my experience, dark armor brings about the same level of survivability as rad armor does, the difference being that dark is way more endurance heavy.
Insert standard play what sounds fun rant here.