Blackout is good for PVP, it's like Impact that way. Stunning other players totally messes up their game. In PvE a LOT of people argue that talents like Impact/Blackout make it harder to pull since you can get a stun out of an attack leaving the mob in range of other mobs that could wander into the stunned mob and take it as a sign to attack.
PERSONALLY, I see it as DPS opportunity in PvE. People who are aware of aggro ranges can pick and choose their targets easily in every area save maybe Outland where mobs are packed so dense. In groups you wouldn't be the one pulling anyway. So as a mage I choose my targets carefully and impact totally works to my advantage. I suspect Blackout could equal the same results.
Spirit Tap is awesome tho, if I had to choose over the two I would always pick Spirit Tap. The build I posted doesnt leave a lot of room to move points around to accomodate both really. If you were group/raid focused you might shift things around to push for reduced thread and fewer crits.
As an aside:
As you get to higher levels and resilience comes into play, a concept I STILL dont fully understand, you may want to make slight adjustments to improve spell penetration or lowering target resists to your school of magic. I know my 10/48/3 build uses the 3 in ice for elemental precision for example. Once youre that far tho you won't need advice from a dumb fire mage