I joined WSI in 2003. IIRC, "dark cloud" or "pants pissing dark cloud" began being used in 2005 to mock the people who were cynical despite the Sox remaining in first place throughout the year. The "dark clouds" were out in force in August and September 2005, when the Sox were playing .500, the Indians were playing .900, and the lead shrunk from 15 games to 1.5 games.
IMHO, the proper context to call someone a "dark cloud" is if/when he/she is crapping all over a first place club, or one that has a winning record and is within striking distance of first place.
Expressing cynicism during a losing season is entirely appropriate. Just don't overdo it.
The universe is the practical joke of the General at the expense of the Particular, quoth Frater Perdurabo, and laughed. The disciples nearest him wept, seeing the Universal Sorrow. Others laughed, seeing the Universal Joke. Others wept. Others laughed. Others wept because they couldn't see the Joke, and others laughed lest they should be thought not to see the Joke. But though FRATER laughed openly, he wept secretly; and really he neither laughed nor wept. Nor did he mean what he said.