Blood is more reliant on physical damage and crit/ap value then frost is.With more crits/ap from raidbuffs and sunders on the mobs, blood would likely be closer to frost in your case. Also cent of blood will give more rune strikes to the blood spec.
Tbh i dont realy know how blood vs frost threat is in 3.1. It depends on so many things. What gear do you have, what buffs do you have and do your warrior bother to sunder the mob etc.. It's still early in the ptr and talents are likely to change so i would rather wait untill it hits live before making to much conclutions. Or head over to Elitis Jerks they have prolly done some excessive dps testing