Heal through the Black Blood
As I'm sure you know, Morchok has regular Black Blood phases where he poisons the ground and you run behind the rock pillars. The Black Blood phases happen in all three difficult levels of the encounter. Your raid's DPS goes down during these phases because all of the melee DPSers have to run away from the boss. With a bit of planning ahead of time, you can fix that.
The rock pillars block line of sight for healing, but you can sit just on the edge of the pillars to fix that. You're still dodging the blood on the ground, but you're also able to heal melee DPSers who are attacking the boss. If your melee DPSers have some defensive cooldowns like Feint or Anti-Magic Shell, you can heal them while they stand in the blood.
The melee DPSers will take quite a bit of damage, but it's nothing that Divine Light spam can't handle. I pop my Divine Protection for the Speed of Light bonus to get behind the pillars quickly to set up. I use Beacon of Light as well, which allows two melee DPS players to stay in full-time via my healing. If you can get the other healers in your raid on board, they can help out as well. There's no reason for your DPSers to run out of the Black Blood once you know how to heal through it.
Speaking of healing through damage
You can use the same technique as above to cut out an entire phase of heroic Zon'ozz, and it can even help your group beat the enrage timer if you're having problems there. Rather than using a normal four-bounce rotation, you can have your whole group stack up after the third bounce. One person bounces the ball in for the final phase, and instead of running out and attacking the eyes and flails, you simply stack up and burn the boss.
Save your Divine Protection, Aura Mastery, and Divine Favor for this phase. If you use Divine Favor to help mitigate the first black phase, it will be up again for the final phase. Simply spam Holy Radiance on your stacked group to ensure their survival, and enjoy all of that extra damage that your DPS is pouring on. I actually find that turtling through the final black phase is easier than chasing everyone around as they run from target to target.
Seriously, just heal through everything
You can use this same tip to help your guild meet heroic Yor'sahj's tight enrage timer. Once the boss is under 20% life remaining, try having everyone ignore all other mechanics. They don't even have to swap to oozes. With your DPS focusing on the boss, he'll drop quickly. I simply heal through the adds' damage and use potions or Divine Plea to handle any Mana Voids that are used.
By using all of my remaining mana to simply burst heal through the final portion of the boss' life, I'm essentially trading healing for damage, which I'll gladly take any time.
If you can't heal it, soak it
Hagara shoots several beams of Ice Lances at raid members. If a melee class is hit by one of these, their damage goes down. As a healer, we can help out by soaking these beams up so that they never reach the melee players. You should be eating as many Ice Lances as you can. On normal, you can handle a whole stack with Divine Protection. On heroic, volunteer to be a soaker. Holy paladins are uniquely suited for the job with our Divine Protection ability, which should be glyphed at all times in Dragon Soul. You can also pop Divine Shield to completely negate the Ice Lances for several seconds.
You can also assist your raid by aggressively soaking the Twilight Barrages on the Warmaster Blackhorn encounter. In fact, you can even solo soak a Barrage on heroic difficulty if you use Divine Protection. Every time you soak a Barrage, a DPS player gets to stand still and get some more damage dealt.
If that doesn't work, shoot it
Finally, I pull out my Denounce spec for a couple specific purposes in Dragon Soul. First and foremost, I use it on the normal Spine of Deathwing encounter to break players out of the Fiery Grip stun. I also assist burning down the Amalgamations when I'm not otherwise busy.
I also alternate between Exorcism and Holy Radiance for the first 40 seconds of Ultraxion, which lets me deal some reasonable damage while keeping everyone alive. I'm not looking to deal 20k DPS as a holy paladin, I just want to help my raid move as quickly as we can. I will often finish off the final eye stalks when we're facing heroic Zon'ozz to let everyone else get back into position. If I'm sitting idle, then I'm wasting time. I am actively looking for ways to turn my idle time into increased uptime for the DPSers in my raid.
The Light and How to Swing It: Holy helps holy paladins become the powerful healers we're destined to be. Find out just how masterful mastery healing can be, gear up with the latest gear, and learn how to PvP as a holy paladin.