Back when b2evolution showed referral stats publicly a lot of spammers sent fake hits to a site to get listed as referrers (and get backlinks to their sites). So, the blacklist looks at HTTP_RERERER and if it matches a url on the blacklist, the hit is turned away. The referers from spammers are faked, so you're not turning away any real readers. Just spam hits.

an other site that get into the 'blacklisted' onces : your own site.

If you have a link from one post to another, that referrer will get into the blacklisted once...