It reduces the damage you take, as well as increasing your aggro each time you block. It's pretty nice for tanking, but the 80 hp applies before damage reduction from armor/buff so it only actually blocks like 30 damage or so.
this blessing is mainly for when AOE tank grinding, its not worth using for the damage or reduction against 1-2 mobs unless you got too many pallies in group..
I use it anytime I am fighting multiple mobs or when fighting elites. It's worth having although if I am fighting a single mob I usually go with BoW.
This weekend I was duoing the cauldron runs in WP with another pally. We were both prot so we just waded in and fought every mob in the area around the cauldron at the same time. It was nice because with 2 pallies we could have both BoS and BoW up at the same time.
I will use BoSanc most of the time unless I am worried about using up too much mana. As a full prot spec it makes a noticeable difference when I have it up. Naturally I will almost always try to take on multiple mobs at once when soloing.
Also, if you are MT for any of the instances then BoSanc is wonderful. I always notice if I fail to renew it when it expires. There will be one pull that I seem to take more damage and had to work harder holding agro. I'll look, and sure enough, my BoSanc will not be there anymore.