The blackest night changes in 7.0?

[ForteNightshade]

I don't think that is a significant issue. It's only an issue if you aren't very good at judging when and how to use it and against which enemies.

This is not entirely accurate. In PF, you'll often not know how people will mitigate (or if they will at all), which can impact whether TBN breaks. A prime example of this was P2S where tanks could eat one of his cleaves without getting a damage down if they zeroed the damage. As a DRK, if the other tank tossed you their short CD (Nascent/HoC/Intervention) without telling you or the healers overmitigated, TBN wouldn't break. If you didn't use TBN, you couldn't zero the damage, thus were forced to disengage and lose 2-3 GCDs. You essentially had a guessing game whereas none of the other tanks had to care.

P7S is another fight where you actually have to strip off your entire gear and reequip everything at 1-2 seconds just to make TBN weak enough in the opener to break. If you don't, you pull with Edge and just accept a GCD loss. This is partially made up by the knock up stun cutting into your GCD. So it's manageable but still goes to show how poorly thought out this interaction can be in certain fights.

A far more common spot is the opener. It's not uncommon for the boss to auto someone else when pulled, which can result in the MT not taking enough damage before TBN falls off, costing you the aforementioned 460 potency in a pot window. That's not a small amount of damage loss.

Why do people insist on erasing every last trace of uniqueness? Aren't tanks homogenized enough?

There's nothing unique about this interaction. You're literally tossing out pointless TBNs so you'll maybe squeeze one extra Edge in one of the burst windows. It's such a minute gain you'll still orange without even doing it. On the flipside, if TBN doesn't break like in the scenarios I highlighted above, especially in the opener, you'll lose 460 potency. What makes it frustrating is you have zero control in these situations.

For a rough comparison, this would be the same as calling White Mage throwing out pointless Raptures just to get a Misery in burst windows "compelling gameplay." I get the whole "rawr, homogenization bad rawr!!!" but being "unique" for the sake of it isn't good design. I'm all for giving tanks (and jobs in general) more class identity, however let's not cling to jank design in a desperate attempt to have "uniqueness."

the question shouldn't be "why does TBN work the way it does?", it's "why is DRK such a shallow and terribly designed job that the only thing it had going for it up until 6.4 was damage?".

Now this, this is the question we need to be asking. Dark Knight's entire "identity" has been "big damage". The second the other tanks, specifically Warrior and Gunbreaker caught up, it immediately died. Kind of highlights why the whole of Dark Knight's design hasn't been great.