Last week I got a fading sun coral from my local LFS. It is yellow and looked very much like the condition of your black. I have had amazing results by pampering it royally.
I don't feed the rest of the tank until the corals have been fed, this helps keep you from overfeeding.
First off, I watch it until I see even the merest sign that something is opening out within the coral. Then I chum the water (drip about 10 - 20 drops of zoo plankton into my 24G nano). Within an hour I notice that the sun has opened even more.
First I dip about 1/4 Cup of tank water into a jar to mix frozen mysid shrimp, frozen squid, and frozen brine shrimp together. This makes the water too cold to feed the coral so warm it up (I just dip the container holding the shrimp mixture into a bowl of hot water and stir with my finger to get to the right temp). Mix the amount you would normally feed your whole tank but use it for the corals first.
Next use a ziploc bowl that is about half again the size of your coral and deep enough to completely submerge the coral. Submerge the bowl, take the coral and put it into the bowl, pull the bowl out of your tank and pour off enough water so that you can float your bowl in the tank. This keeps the coral at tank temp for feeding. (Don't pull the coral out of the water into the air.)
Pour the warmed shrimp mixture directly into the ziploc bowl containing the coral. Use a turkey baster and move the water around the coral. You will see the coral grabbing pieces of shrimp or squid. I usually leave the coral in the bowl for at least 10 minutes stirring the water about every 3 minutes or so. When the coral has finished eating submerge the feeding bowl into the water, remove the coral to it's original spot and let the rest of the tank have the remainder of the coral's dinner.
In only a week there is a HUGE difference in the ailing sun coral. If you don't really want to get rid of the beautiful corals give this a try! My tank isn't showing any signs of overfeeding and the sun coral is really reviving.
