Here are some blocklists that can be used by Transmission:
With Transmission 2.12 (11412), you can specify the URL to the blocklist. The blocklist that used to be downloaded is:
There are more paranoid blocklists at Bluetack’s website (you want the eMule version with the .gz extension). Each is updated weekly.
Normal IP Filter: http://www.bluetack.co.uk/config/nipfilter.dat.gz