When expecting char encounters you should take dual P/W's with 2x Defensive Anthem and 1x "Watch Yourself!" + "Shields Up!" and a debuff/interrupt N/Me, then Mhenlo, Lina, Herta and Zho. That is certainly not the only working combination, just check that there are enough blocking skills to shield your party from charr warriors and rangers.
Blood Washes Blood is indeed a long mission. However, if you start from Longeye's Ledge you'll be able to reach the shrine much easier than if you started from Sifhalla (there's only one mob of Jotuns in Bjora that you have to kill and there are fewer vaettirs between the portal and the shrine than between the northern gate and the shrine).
When the actual mission starts, there are three points where you have to be particularly careful. Immediately after entering Bjora you must take your time and watch patrol routes, then pull them one at a time to a safer place to avoid multiple aggro. Around the bear shrine the boss mobs patrol around and depending on your approach angle you might have to pull them downslope to prevent other mobs' monks healing those that you fight. Finally, the boss mob at the end of the mission is pretty tough. Monk boss, dominator, nuker, 3 warriors. The best strategy is to first lure the nuker's AoE to a harmless place, then group hug the boss (he spams heal area) and kill the dominator (who has the hard rez), then the nuker before his spells have recharged, then the boss and finally the warriors that you could safely ignore until then if you had WAM and DA running as suggested.
Yep it is easy to run through without aggroing anything. The vaettirs around the shrine you need to pull a nice long way away. It is far to easy to aggro 3-4 groups at once.
For the BwB mission itself, make sure you have UB on your skillbar to start with. Then you don't have to fight all the way up and over. You can just head more or less directly east. The groups in the final area seem to be much easier to deal with coming in through the bottom gate too.