So I've tanked for a while as frost and found it very easy to use in heroics. But lately I've wanted to try blood tanking. Single target aggro is a breeze but when I come upon a group of 3 or 4 I find it a little hard to keep aggro on all the mobs. Here's a link to my armory:
My typical "opener" rotation is to DnD > IT > PS > Pest. But at that point all my runes are on CD and unless I blood tap I'm useless for a few seconds. Sometimes that dead space is OK, other times I'll have a DPS going nuts on a mob and peel it off me. Of course, I'm able to taunt it back. The usual suspects are Ret paladins and warriors.
I'm wondering if there's either something I can do differently in regards to my spec, my glyphs or my "opener" for heroics.
Yep. That's blood aoe tanking. It is a weakness if you don't outgear the instance or your group. For trash, you should start using a dps item or two to increase your AP. You can also swap out the glyph of RS for DnD. That should give you that extra bit of AoE threat you need.
Also experiment with not using Pest in the first round.
1: DnD -> BB -> IT -> PS
2: Pest -> BB -> DS -> DS (DS on targets you're losing threat on, use Tidy Plates: Threat Plates addon to see this clearly)
3: BB BB BB BB BB BB (or tab-spam HS, whichever is more appropriate)
Haven't tried it myself, but have heard of others doing this. A little more snap AoE threat up front at the cost total threat gen the first two rounds.
I've got my fingers crossed that in Cataclysm Blood gets a HB or Shockwave or some other snap AoE ability.