Unholy tanking is not viable atm. Use frost for mitigation and Blood for effective health. this is until an effective build has been found and some tweaks done. But currently the damage you take as a unholy tank is just too high to be good for you. but this is also endgame tanking we are talking about. If tanking naxx you could get away with an unholy tank build, but your threat would be horrendous if you have decent dps with you.