Was it a real candidate or was it like we could have said "Bush wasn't unopposed, the Democrats ran Kucinich? 
Ferrer, of course, is not Kucinich. Under normal circumstances he could be a plausible candidate, though exciting he is not either. But long before the election - in fact, before he won the nomination - the election was indeed effectively conceded by the Dems. The problems is, that most Dems have nothing against Bloomberg - ideologically and culturally he is one of them, anyway. So, even though the party went through the motions, a lot of its activists/donors actually supported Bloomberg. Even those Dems that, due to their position in the party, had to publicly support Ferrer or even campaign for him (e.g., the Clintons), tried to do this as unconspicuously as possible - they really, honestly, truly actually prefered Bloomberg.
The Dem problem there is that to win the Dem nomination one has to "rise through the ranks" and "pay the dues", and the result is, usually, boring. The Rep nomination, in contrast, is almost there for taking by an ambitious outsider (since its value for a boring insider is exactly zero - he'd never win), and the result is frequently interesting. Thus, the Dem advantage is sometimes counterbalanced by the quality of Rep candidates. This works only for the mayoral election, of course - not a single extra Republican council member was elected, despite the mayoral landslide; no coattails watsoever.
