If you couldn’t hold threat as frost in wrath you probably couldn’t do it as blood either. Rune strike was important in any spec you wanted to tank with and did a high amount of threat and wasn’t on the gcd. You just made a macro for all of your attacks so you didn’t actually have to press the button. Also when the change to Icy Touch happened due to the dodge nerf in ICC which resulted in less rune strikes, they changed the threat modifier to be through the roof and frost had more access to frost runes that no one could out threat a frost dk.
The reason why Blood was chosen was because people were solo tanking heroic Lich King as blood. I remember them talking about why they chose it and even explaining stuff about having a tank that was different than the others through healing the damage done to them than mitigating it, though we all know avoiding the damage outright is better than trying to heal it (look at disc some expansions).
I absolutely highly doubt that Blizzard is going to do a spec within a spec ever again. Every iteration of a spec inside a spec has been removed or in the case of feral a 4th spec for tanking was created which is Guardian. Everything points to them not wanting specs inside of specs, or roles within a spec that is a different role. Gladiator I’d gone, dark apotheosis is gone, guardian is a separate spec. And then specs within specs are gone, single minded fury, 2h frost, ww monk 2h.
If any of these are to come back then Blizzard has to change internally in design philosophy.