It depends what level you're talking about as to whether messageboard/blog culture affects album sales.
If you think how totally oblivious we dissensians are to bands/acts being hyped on a message devoted to, say, emo (to pick a niche genre at random) ... you can see how marginal messageboard culture is in terms of wider reach. The reason I don't know what emo bands are hot is because I've got no interest in that genre - if I did, I'd seek the information out. If that rule doesn't hold for pop/rock/indie, it's because it's part of a mainstream culture that people absorb passively. Britney Spears and Pete Docherty are unavoidable unless I walk around with a sensory deprivation device on my head, despite my having next to no interest in thier music. Albums like Burial are never going to enter mainstream culture in the same way, and no amount of blog/forum action will change that.
However, what it can do is eliminate global boundaries - so only a very select group might be interested in Burial in the UK, but if you extrapolate that to every country with internet access and collect all those fans, you're talking about a pretty sizeable number of potential sales for an independant label - maybe 30,000 or so. Most of those are people who indie labels would have absolutely no hope whatsoever of reaching without the internet.
Major labels use mainstream media channels to try and seduce or hoodwink people who aren't really interested in thier acts into buying the product anyway - think of all the Jamie Cullum-type CDs people will get bought for christmas and only play once. Indie labels operating in niche genres can't afford to be anything like as ambitious - thier main goal is to seek out people who already want to buy thier record, and simply let them know it's available. For that purpose, forums & blogs are brilliant.
However, once they get through to those people, they encounter two massive problems - both caused to some extent by the internet. The first, obviously, is how to convert that interest into sales; that is, how to get people to buy the record instead of pinching it in some way. Downloading is a really complex phenomenon, but it does seem to affect indie labels more than majors - partly because thier customers tend to be more techno-savvy, but every year the mainstream catches up and this becomes less of an issue. A much bigger factor is that majors spend a huge proportion of thier marketing budgets on point-of-sale stuff and in-store advertising - that is, marketing that will reach people only after they've decided to buy something; the challenge then becomes to guide their eager wallet towards your artist, not someone elses. Contrast to the indie consumer, sat at home on the net, surfing soulseek and chatting to thier mates who maybe already have the record they want, which is the situation most will probably be in when the message reaches them. Far more difficult to turn that into a sale than someone wandering around HMV with a £20 in thier hand. But the defenders of download culture are also right to point out that the general health of forums and blogs, and this whole global communications network that can be accessed for next to nothing, depends on the free flow of music. So a lot of indie labels find that what the net gives with one hand, it takes with the other.
The second problem is that the internet seems to have accelerated everyone's attention span, so that artists only have to be out of the public eye for a few months and people totally forget about them. Think of an artist's public persona as being like a big, heavy flywheel, and when they are in the public eye that flywheel is spinning round quickly. Major labels can afford to let the wheel slow down over time while the artist is out of the limelight recording or whatever, then when they need to they can give it a mighty heave, in the form of a massive marketing spend, to get it back up to top speed again. Indies can't afford to do this - they might have given everything they've got to get the wheel spinning in the first place and they must make sure it doesnt slow down, so they keep having to nudge it all the time to maintain a steady speed. In the real world, this means constantly releasing new music, updating blogs, touring, etc - not really expensive, but very draining, and there's a real risk that you'll just run out of fresh things to say and do, so the audience gets tired and drifts off. Lots of dance labels suffer from this, and only the really clever ones manage to get round it. People are naturally attracted to novelty, and that doesn't make life easy for labels trying to build a sustainable long-term business model.
For ten years I've been waiting for someone to come up with an on-line indie music model that addresses these problems and manages to harness the benefits of the net for underground music whilst minimising the downsides, but nothing has come along so far. One day someone will probably crack it, but I think people are starting to realise that these traits have much deeper and tougher roots than was previously thought.