Few reasons actually.
1) Blackfire is known, at least by some, so we can count that out. At least that the organization exists, anyway.
2) 629+ people, call it even 650 people, is a relatively small number when regards to other criminal groups such as Black Sun, the Hutt Cartel, so on and so forth.
3) Citadel itself is the remnants of the original Citadel organization; few if any know Stark's real name. They've been gathered on and off since the Clone Wars, reorganized, consolidated and even suffered a mass purge at one point to form the loyal, strong core that is currently Citadel.
4) They don't operate from a single location, which makes it easy to keep quiet.
5) Citadel's contacts include placement in the Bounty Hunter's Guild, the Hutt Cartel (small placements) and even among the Empire's mid and upper echelon.
6) Secrecy is key, for the entire factor. A multitude of small raids here and there, such as at Telos IV and small takeover actions have fed the group over the years, as well as money laundering through a number of fronts.
All in all, considering it's been gathered over time, it's not all that hard to imagine such a small group maintaining secrecy.Added (09 Jul 2012, 3:32 PM)
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Also, if one considers funding from such sources counterfeiting, prostitution, arms trafficking, direct theft and bribery and money laundering, it's not all that hard for an organization built like this to amass power without ever actually being truly seen.
