I received this message back when sending out the word-a-day. I understand the gist of it, of course, but can anyone explain to me more specifically what 'blackhole site' and 'blacklist' mean in this context?

Mail from [IP address] refused by blackhole site [site name].net - have your email admin check http://[sitename].net for information on this blacklist

It means that one of the email's recipients has a script that checked the originating IP address of incoming email (i.e., your IP address) and that it refused it because the IP address is listed on one of various blackhole sites. It might be that your ISP is lax about being an open relay of spam, and that somebody has listed their IP addresses (one of which you use) with the blackhole site. Going to the site should have more info and/or an address to appeal your case.