Dark Knight is still the magic tank. Its attacks will still be spells and Dark Mind still mitigates magic. Dark Mind has always been inferior to other cooldowns which can mitigate both, but fortunately most bosses you'd need it for deal magic damage and some mob packs spam magic casts.

Sheltron should always have blocked magic because the Heavensward trailer depicted the Warrior of Light blocking fire (a type of magic damage) with their shield, proving it is realistic to block magic with shields in FFXIV lore. For whatever reason, blocking was not actually adjusted to be able to do that in Heavensward.

Paladin has always had an element of casting spells simply because it used to cast Cure, Stoneskin and Protect in ARR. In HW it got its own versions of these in Clemency and Divine Veil and evolved from there.

I don't think any of the tanks will have a problem with Rampart, their 30% and their Sheltron/TBN/Inner Beast/Heart of Stone, so it's better to view Dark Mind and Camouflage as bonus cooldowns.