Crater made an interesing observation in this post, which is that DRK's design niche is tanking actively. Ever since reading that, I've changed my opinions somewhat, as he makes a very good point.
TBN is a powerful ability and one which really isn't like any other tank cooldown. The main things that need changing about it are its dubious effect on our DPS and resources, and people have suggested dozens of ways of fixing this, here are just a few examples:
-Reducing the mana cost
-Procing a DA effect if it pops
-Reducing the Blood Gauge cost/increasing the potency of Bloodspiller to make the gauge generation worth more potency
-Having TBN reduce the recast of various offensive abilities (C&S, Salted Earth, Delirium, etc.)
Nowadays, I'm fine with TBN being our "defensive Dark Arts" of sorts, however, even still, DM or Shadow Wall need fixing. The author of the post linked above believes the weakness of Shadow Wall is balanced by TBN (in much the same way as the old DD+Reprisal balanced it in 3.x), but that DM should be brought up to speed and work on physical damage, as well as receiving a duration increase, which I think is a good compromise, and one that, approaching 4.1, we are overdue for.
A cooldown that only works on magic damage in the current climate (PLD is able to block magic, WAR still as capable of tanking magic damage as it ever was) is just dated. Its powerful in o1, 2, and 4s, but that's it (and those encounters are clearly designed with it in mind, and indeed, the entire raid tier, as the only fight that has a physical buster (o3s) is designed in such a way that you have a ~30-40% extra mitigation CD in Awareness). Everywhere else it is extraneous, because magic damage is not present in this game in the way physical damage is. A cooldown like Raw Intuition may have the same problem from the other direction, however because of the ubiquity of physical damage, it is useful in the vast majority of content.
If this raid tier, and o3s in particular, proves anything to me, its that a change to DM surely is imminent, as SE will be unable to design a fight with a frequent physical buster that isn't a critical hit in the future, which seems like a ludicrous design restriction for them to enforce against themselves to accommodate a single job, especially when so much of the job's playerbase is acutely aware of said accommodation, and hates it.
What I'm about to suggest likely will never happen, but it would stave off ever-present fears of homogenization (and avoid the awkwardness of a cooldown called "Dark Mind" mitigating physical damage somehow):
Dark Mind: Reduces magic vulnerability by 10% for 15s. Recast: 50s. DA effect: Increases reduction to 30%.
Dark Shell: Reduces physical vulnerability by 10% for 15s. Recast: 50s. DA effect: Increases reduction to 30%. Shares a recast with Dark Mind. Has Shadowskin's old animation.
In a word, yes, it was too OP. Or at least that's what SE thought. Imagine if WAR's old Zerk-Bloodbath-Overpower spam not only leeched HP but also TP, then you have an idea of what we enjoyed back in the day.
I'll admit DRK's AoE tanking rotation is far more complicated now. However, one thing hasn't changed: DRK needs big pulls. Everything hinges on it: Blood Price, Quietus, AD, even TBN. That having been said, there is a correct way to do it, at least based on my experience. Based on some key statement you made in your post, I don't think you're pulling enough, which is not a new problem for DRKs, as I frequently recall telling others to do this in 3.x as well.
There's a basic foundation you have to lay first: Pop a CD, Salted Earth, and BP. That's step 1. The next is TBN. If you've pulled enough it should pop quickly. This first burst of Blood is for Delirium. Delirium BP will sustain you very nicely. BP may have been nerfed hard but it is still very effective in large pulls. After that its just a matter of DA-AD ->Quietus ->TBN.
The key is to watch BP and adjust your play accordingly when it falls off. While its running you shouldn't have any problems. When it does fall off, its recast has only 9 seconds left (because of Delirium), so that's basically 3-4 GCDs, which I usually spend using HS-SS-SE-(DA)AD. After that you have BP and Salted Earth again and everything starts over. It is really complicated, but after most of the complexity of our job was removed in this expansion, I'll take my challenges where I can find them.
All this having been said, there are still key perks to AoE tanking on DRK that the other tanks don't have. In order to recover the resource PLD uses to AoE, they have to Flash and wait for TP ticks, which means doing no damage at all. We have cooldowns to do this while we continue AoEing, or at worst, we have to switch to a single-target rotation for a few GCDs, which is still damage. On WAR, you can recover your AoE resource through single-target, but that is it. There is no way to generate gauge or recover a significant amount of TP while AoEing. On DRK you can do it all simultaneously, so I'm fine with enduring added complexity in exchange for that.