My impression is that police officers will work with the information that is readily available rather than jump through legal hoops of fire.
They might try to get the information "for free", for example by contacting and putting pressure on known members of the board, making threats and allegations which would will probably not be upheld in court, but might scare people enough to volunteer the information. This is a very cost-effective method and probably the most common.
Ofcourse they could get a court order to get hold of the information from the board staff; procedures may vary in different countries but this method will work everywhere. They might address our provider or an (ex)admin to get this information. I expect that on receipt of a duly authorized order, any party involved will cooperate as required and hand over the requested information. Who is going to risk a few years for destroying evidence, obstructing the process or passing information on to the accused? I wouldn't. I see this as a theoretical situation as minor offenses do not justify the bureaucracy associated with the legal path.
A much better method would be to place a tap on the network without us knowing: a court order for this is a routine job. Tapping is especially easy in Australia since there are only a handful of cables to the rest of the world and these locations are equipped with sophisticated packet sniffing equipment. If they would really want to pursue a warrant, this will probably be the method of choice, as it gives the additional advantage of being able to operate with the Bluelight community remaining unaware.
But imagine the paperwork and man hours involved with this, especially if it involves getting statements and cooperation from admins overseas: we can post and chat as if the server is next door, but for law enforcement it will be a lot of wasted time to receive proper authorization and collaboration with colleagues from other countries - so is Bluelight being approached by law enforcement a possibility? Yes, ofcourse. Is such an unattractive measure likely to happen soon? Not for drug possession charges. Not even in Australia. For outright dealing? Well... "purrrrhaps, perhaps, perhaps..."