The way I do BM is tank the Rift Lord near the portal, ignore the adds, and let the add handler kill them.
The Black Morass is EXTREMLY easy to tank, but DPS have to be fast. Time is of the essence. If your healer can't drink in between fights, he's in a trouble(unless he's well geared and so is the tank), if fights last too long, the add handler will run out of mana, if the add handler doesn't kill adds fast enough, either Medivh get some damage, or someone from the party will help him reducing damage done on the boss. If a portal lasts too long, ANOTHER will open.
It can be summarized as a DPS race. Your job as a tank is to cause maximum threat on the Rift Lord, so others can cause maximum DPS on it. The fact that you are a Druid, is a nice help there, as you have arguably the best single target DPS as a tank (not sure what Paladins are capable of when tanking one mob, on several mobs though, their damage is impressive).
One more advice, be sure to bring someone with offensive dispel(Priest for Dispel, Shaman for Purge or Mage for Spellsteal) to dispel the haste buff the second boss gives himself. The boss will debuff the tank with -10% healing recieved, which can now stack 7 times(Last time I did it, it was 10, so some things might have changed), if the boss is hasted, he will stack it 7 times(been with 10 on me on bad groups, so 7 should be easier for him to get), if the boss is constantly Dispelled/Purged/Spellsteal-ed then it shouldn't pass the 2-3 stacks.
Hope it helps, and good luck with the Black Morass, tell me how it's working for you,
Yuval R.