I noticed that in a raid recently, Right after I taunt a mob, I have initial Aggro issues. Usually my threat starts at 2-2.5k tps, then eventually jumps to 5-7ktps. I think its because they unleash hell on the mob the minute I throw a taunt, but Now I'm not sure. My rotation is: IT,PS,DS,HS,HS,-dumps>HSx6 Below is my armory:
Your expertise is a little low for Blood, try and push it up to ~26. Take 2-handed weapon specialisation, it really increases your threat. Subversion is a better point investment than Sudden Doom for threat, Death Coils are a very low % of Blood threat. Swap out your D&D glyph for Rune Strike as that's our primary threat tool. Oh, and macro Rune Strike to everything:
Tell your DPS to give you a moment to get aggro first.
Put a mark on the target you are focusing on.
Switch targets occasionally to spread the aggro around.
Blood tanks do not shine in AoE situations but it is more than possible to keep aggro on a group of mobs provided you are following the above suggestions.
If you are speaking more for single target threat, ask your DPS to wait until you have both of your diseases on there before they open up. Then at least by the time their attacks land, you have probably gotten in a Rune Strike and/or Heart Strike.
A lot of DPS seems to think that the days of "let the tank get aggro first" are over, but this is most certainly not the case. Premature DPS is always the number 1 reason for losing aggro, aside from just a crappy tank. But it's hard to consider a crappy tank a tank at all, really. More of a failed DPSer. :P