are the leaves fuzzy to the touch? thats what it looks like and i feel like that could be important... You might also try washing all the leaves thoroughly, checking every bit for some wierd bug...pests are not my specialty :)
are those thorns or leaf stems (petioles)

Leaves aren't fuzzy, I've found a few mealybugs but I've picked them all off. Those are all thorns btw.

mealybugs will cause the sooty mold. wash them off with rubbing alcohol on a q tip. Check your plant about once a week because it's impossible to get all of them. It will take about a month of once a week inspection to get them all. Mealybugs will cause the leaves to drop off too.
Also make sure it's not too cold or too wet.

I have the same problem on a few, but not on all of my trees. They are indoors next to a window that gets bright sun.
The one from Four Winds is dropping leaves and has thorns but still has blooms. The one from Stark Bros looks fine.
They are indoors, and I have been watering regularly. I also have a little citrus group (orange, lime, lemon) from Gurneys. One has leaves, the others have spikes and no leaves.
I was thinking of repotting bigger ones in a grainy mix (if I can figure out what that is) They are about two feet high. I'd post pix if I could figure out how. Help! I am in Yorktown VA