This would be a great idea, except for the fact that WAR is already is the tank that you describe.

If you were creating a new game from scratch, would it make sense for a darkness-themed knight to have life siphoning/manipulation abilities (like Warcraft's Death Knight)? Absolutely. Would it make sense for a Berserker to ignore pain and shrug off damage (like the quintessential DnD Berserker)? Absolutely.

One of the foremost challenges with any MMO rework is that any time that you rework a job, players will come to you and complain that this isn't the job that they remember. So any successful rework on some level is going to have to be faithful to previous iterations of the job, even if they're difficult to reconcile with each other. What you've essentially done is taken a problem (players are not happy with DRK's design) and translated it into an even bigger problem (rework two jobs, with the risk of even more players being unsatisfied with the result).

My recommendation is this. Forget about lifesteal and trying to swap places with WAR. Bad things lie in wait at the end of that timeline. Instead, deliberately move DRK in a direction that allows it to differentiate itself from the other tank jobs. Right now, DRK is the job with the least amount of self-healing. That's perfectly fine. Commit to damage prevention then. You remember what I said about shrugging off pain?

'So as I was saying, that heretic was fighting like a man possessed. Even after he took several wounds, he showed no sign of pain─though there was no mistaking the blood.
As the fight wore on, it began to soak through his armor, spreading to every ilm of his body. But when it began to rise and envelop him as a mist, I realized it wasn't blood, but something dark and twisted...'

It's in the lore. That's the design aesthetic that you should be looking at. It may mean that your fantasy preconceptions of Berserkers and Blackguards are reversed, but it shouldn't matter as long as each job gets the opportunity to be unique and desirable in its own way.