If you're in any capacity serious about Blood PvP, you have Unholy Blight. It's not a "hybrid", it's the go to Blood PvP spec.
Unholy and Affliction both absolutely destroy Shadow with minimal levels of skill required, it's some of the hardest counters in this game. At higher levels of play, Shadow also has very unfavorable odds against Sub rogues and Frost mages. Not really what I call spec with least counters.
The only spec in the game that comes close to having good odds against everything is Frost mage and only at very high levels of skill. Destruction warlocks, Unholy DKs and Feral druids are close second. Rogues would also be there if they didn't get countered so hard by anything with plate and shield or tanking stance. All this is assuming a dueling setting, If this is world PvP/BG 1v1, rogues take over the spot of Destruction warlocks as one benefits from its opponents not being prepared for an opener and the other one requires preparation for optimal performance, something you don't have time to constantly do while moving.