First of all - both specs are viable at this gear level and each of them has their place. I have both specs at the same time in naxx and regularly swap between them depending on the situation. For heroics I would almost exclusively use frost, it's not even contest there (unless vs boss with a bad group that will probably die and I'll have to solo).
For naxx, I typically use blood for long fights where tanks don't have to take turns taking boss and I can make use of healing (4HM, Heigan - rune tap is amazing after dispell). On most other however bosses I use frost, since I'm either on adds (Howling Blast is amazing, KM amplifies it even more) or there's magic damage involved or have to rotate cooldowns with other tank.
There's lot of misinformation going around regarding blood vs frost (probably because situation changes with better gear), first, compare passive defenses you get from talents (using typical builds):
Frost:
* 10% dmg reduction Frost presence + 5% blade barrier
* + 3% miss chance
* Good magic dmg reduction - Acclimation (very useful on multiple naxx boses)
Blood:
* 8% dmg reduction Frost presence + 5% blade barrier
* +3% stamina
* Will of Necropolis
* Weak magic dmg reduction - Spell deflection (doesn't work on AoE, low parry % in naxx gear = almost useless talent atm)
As you can see, outside will of necropolis safeguard, frost tank will simply take less damage in all situations (passively), especially on fights with magic dmg.
Then compare active defenses (that differ):
Frost:
* 2min cd -18 sec duration - Icebound Fortitude
* 1min cd +30% armor, 20sec - Unbreakable Armor (glyphed)
* 1min AoE freeze - Hungering Hold (very situational)
* 4k hp Death Strike (assuming 40k hp, typical for geared naxx tank)
Blood:
* 2min cd - 12 sec duration - Icebound Fortitude
* 1min cd - 15 sec - Vampiric blood (glyphed)
* 3min cd - Mark of Blood (~1,6k heal on each hit)
* 30sec cd - Rune Tap - heals 9093,26 glyphed, 8266hp without glyph (assuming same gear as frost, with +3% hp = 41,3k hp)
* 6,2k hp Death Strike (assuming same gear as frost, with +3% hp = 41,3k hp)
Frost is better at short phases of high incoming physical damage (let's say, stacking up enough adds to aoe). UA is amazing for tank swaps aswell.
Blood is better at prolonged fights with less predictable incoming damage, if you can make use of healing. Rune tap is amazing, Death Strike is great, however with current HP pools, it's difficult to squeeze in effective heals. Plus, sometimes you can't even heal when you need it the most, like on Maexxna, Loatheb.