It's not that Oblation is weak, it's that Oblation does not even begin to make up for the weakness of TBN. It seems like Oblation was the devs answer to HoC, HS, and BW but for DRK and honestly it doesn't even begin to compare to how bad the combination (Yes, combination) of using TBN with Oblation actually is when compared to the other tanks cooldowns, especially this expansion. The math has already been done. Those numbers are based on early EW content but the concept remains the same. Especially this part.
And in this expansion, we are taking more damage. Especially in trash pulls (Is it just me or are tank busters weaker?) where DRK can run out of mits if the DPS don't burst them fast enough. Suddenly the 10s uptime of Dark Mind looks more necessary, but it does literally nothing in trash pulls. The fact that people can still sit there and defend TBN and Oblation tells me none of them have actually done the math to see how miserably the skill fails next to the other tanks abilities. It has 2 more seconds than other tank abilities but with 5 seconds (on average, since it is on a charge system) longer cooldown. Either use it back to back and wait 60 seconds or use it between cooldowns (as I think is intended) and be upset you're still taking more damage than them.
It's an uptime difference of less than 1 second leaning in the other tanks favor (Exception maybe PLD since they use gauge on it, but PLD Shield is passive damage reduction other tanks don't have anyway) while they also provide around 4 times the defensive value of raw damage reduction (Math on damage reduction not considering healing and/or shielding such as the heals from the other tank cooldowns or pairing it with TBN, which should be comparable anyway).
Oblation isn't bad, but it also isn't nearly enough. Oblation falls flat as the DRKs answer to other tanks shared cooldowns even when paired with TBN and yet Dark Mind still sees incredibly little usage outside niche scenarios. At least one of them needs to be brought in line.