As I'm sure many people that play Dark Knight can attest, using Living Dead can be a hit or miss. How many times have you used it successfully, slain whatever beast only for you to fall over dead 10 seconds after (with nothing even around to hit you)?
Paladin's Hallowed Ground is very obvious. There is that big bubble shield animation and they take zero damage for duration.
Warrior's Holmgang has the cool chain animation and you can just sit there at one hp and as long as you get healed a little bit, you'll be great usually.
The problem comes in with Dark Knight hitting Living Dead. Once they receive the death blow, there is no obvious animation that "walking dead" is debuffing them. Most healers I've pugged with weren't even able to tell me the correct method to remove it or even knew it could be removed. Of course they just have to keep healing the Dark Knight until it goes away. The mechanic itself works fine. It's just not a very obvious debuff for the dire state it puts the player in.
My suggestion is simple: Make "walking dead" have a obvious animation. No need to change the mechanic itself, just have some kind of animation effect to show the Dark Knight is as good as dead without assistance. How about a animated Grim Reaper giving the dark knight a hug from behind? No? Perhaps some black wings, or a dark black spotlight with skeleton hands grabbing the dark knights feet from the ground to show that the dead are calling to them and going to take them to the underworld soon....etc
TL: DR: For "walking dead" lets get some big obvious animation that shows the dark knight is as good as dead without heals. Heck, even poison turns characters green sometimes.