It was a Tor relay, not an exit node, so I'm still curious why it would have to be blacklisted. And I guess I cannot rule out completely that either my home computer or - more likely - someone I granted access to my wifi has been part of a botnet.

I'm positive that no machine in my home network has ever been part of a botnet; the only explanation that stands up to scrutiny is the Tor relay that was running for a few days as a NAT forwarded service through my router's static IP address.

I wish the faceless entity behind this widespread blacklist service could be identified so I could ask them.