The chimes from the clock send a wave of unease over the guests at the party. When the clock chimes, the musicians stop playing, leaving the chimes to be the only sound throughout the rooms. Poe says that the sounds of the clock make even most carefree people become pale with anxiety; the most reserved people "passed their hand over their brows" out of nervousness.
I do not think much occurs in the black room because the room made the party goers nervous. Any who entered looked so freaked out that there were not many people who wished to actually go into the room. Those who did probably just stood in silence or chatted lightly, as is normal behavior for a party.
It is my understanding that the dreams represent the people at the party in their magnificent costumes, walking around the party, in and out of the various rooms. Poe describes them as freezing where they stand when the clock chimes, just like he describes the guests doing.
The guests are described as being "grotesque," and their costumes were influenced by Prince Prospero and his "guiding taste." They wore clothes with shine and glitter, and the costumes were exciting and strange to look at. Some people wore clothes that did not quite match each other, that were unusual. There is one quote that describes it well:
"There was much of the beautiful, much of the wanton, much of the bizarre, something of the terrible, and not a little of that which might have excited disgust."