Last week I added a three-finger version of Blackbird in the tab archive. Here is another version, where the banjo (in drop C tuning!) plays the accompaniment, based on Paul's guitar version. It is relatively simple: the thumb alternates between the lower strings (mostly the fourth string, sometimes the third string) and the fifth string and a simple counter melody is played on the first string. All chords are just "two-finger chords" (you have use only two fingers of the left hand)
pdf (banjo part only): http://direct.hangoutstorage.com/banjohangout.org/storage/tabs/b/tab-blackbird-(acco-16669-54662312012.pdf
(the text is OK in the tabledit version, but seems to have been distorted in the pdf-version)