I've been working in the email spam space for quite a while now, and I'm very excited about MT-Blacklist (haven't set it up yet because the irritation factor hasn't been great enough, still only a few comment spams a day).

Specifically, filtering on WHERE THEY WANT YOU TO GO is the right approach. It's impossible to blacklist all the IPs because they just get a new IP or compromise one of the millions of Windoze desktops out there on cable modem uplinks. IP-based RBLs for email stopped being a good idea when major ISPs like AOL started getting added. Looking for the call to action in the message is one sure way to avoid this.

And about the graphical challenge, that's not very friendly to many vision impaired folks out there.

I'm hopeing MT-Blacklist will move towards some distributed clearinghouse of URLs with an automatic algorithm for ranking the trustworthiness of the submitters so that we don't have people gaming the system.