Sorry the blocklist change is causing trouble. Here's the story:

The short answer: you'll need to choose a blocklist URL. There are some suggestions in other comments here, and you can also Google for "blocklist" and "url" to find several choices.

The default was removed because the Transmission team doesn't know anything about where its contents come from. I'm not saying the old default was bad... only that, since the Transmission team doesn't know, we don't want to imply an endorsement.

I don't recommend using the old default URL at transmissionbt.com. It's only going to be around until the next Linux distro release cycle -- many Linux users won't be able to upgrade to >= 2.12 until then.