The problem with the block list business model is that false positives have a much lower negative cost to the black list provider than false negatives. Users who find that some spammer doesn't get blocked, will get really annoyed, and make their displeasure known to the blacklist provider (or just switch to some other blacklist). As long as it's not a very high visibility site which is getting blocked (i.e., it's just some startup or a relatively low-profile blog that gets blocked), it's no skin off the blocklist provider's back. So the problem with blacklists like SURBL is going to be a very hard one to fix; the dead hand of economics doesn't really care about irrelevant things like fairness or justice.