From a defensive perspective the Nightblade Tank performs well. You can obtain Major Evasion and Minor Resolve from Mirage and then Minor Protection and Major Resolve with Dark Cloak. This means you gain a good amount of damage mitigation quite easily. Nightblade Tank skills can also apply some interesting enemy debuffs with Major Cowardice with Mass Hysteria and Minor Cowardice with Power Extraction to further reduce the damage they receive.
When it comes to self-healing Dark Cloak works well as a heal over time, it becomes very strong when you are in stationary fights and don’t need to move which makes up for the fact the Nightblade Tank doesn’t have a burst heal skill. You can pair this healing with multiple other skills to stack large amounts of healing over time with the addition of Refreshing Path, Leeching Strikes, Swallow Soul and Malevolent Offering all of which offer additional benefits not just healing.
Sustain for the Nightblade Tank is good but only if you are capable of dropping block frequently for Light and Heavy Attacks. Leeching Strikes is great as it heals and gives you resources with every Light and Heavy attack along with a burst of resources once this skill ends, you could also combine that with the Reave buff from slotting Incapacitating Strike. When you tie that in with the extra recovery from Refreshing Path you have some good sustain. The only problem is this is not ideal for newer players and is more suited to an experienced player.
Nightblade Tanks also benefit greatly from Ultimate regeneration, the Catalyst passive gives a big chunk of Ultimate each time you use a potion which you can manipulate for your own benefit by using Infused/Potion Cooldown jewellery so you can use potions even quicker than the usual 45 seconds. You also have the Transfer passive for more Ultimate.
In the current game meta there is no real use for the NB Tank. In 4-person content you’ll get much more benefit from Dragonknights or Wardens and in Trials, if you have no Nightblade you’d usually see a Nightblade Healer, but you could use a Nightblade Tank then if your group wants one. Aside from that Nightblade Tanks are a challenge for inexperienced Tanks but they can be fun to play.