Well, here's the biography behind the Big 3 (the band who played the Banjo Song).
Remembered today mostly for featuring a pre-Mamas & Papas Cass Elliot, the Big Three recorded a couple of pleasant pop-folk albums around 1963. Elliot was flanked by notable partners Tim Rose (who went on to become a minor but noteworthy singer/songwriter in the late '60s, most famous for performing the version of "Hey Joe" that provided a loose blueprint for Jimi Hendrix's) and James Hendricks. The Washington, D.C. trio were typical of many of the acts working the East Coast coffeehouse circuit in the first half of the '60s, stressing clean and full harmonies, somewhat in the mold of the Kingston Trio. The Big Three were a little (but not much) funkier than the Kingstons, and also occasionally made stabs at a more pop-oriented sound with full band instrumentation. Most of their material, though, was solidly in the acoustic stream, and their repertoire emphasized then-contemporary versions of folk standards, although they did compose some of their own material (which was usually by Rose). The Big Three disbanded in 1964, when Rose left to go solo; Elliot and Hendricks joined Zal Yanovsky and Denny Doherty in the short-lived folk-rock precursors the Mugwumps, which in turn helped spawn two of the biggest folk-rock groups, the Mamas & the Papas (with Elliot and Doherty) and the Lovin' Spoonful (with Yanovsky).
There was a DJ who's name was Leo Blokhuis. He interviewed The Shocking Blues Robbie Van Leeuwen (who composed Venus) and asked him of the similarities to the Banjo Song. Evidently Leeuwen said he dug the Big 3 and was embarrassed about ripping the song off, but also added that all blues tunes sound the same so why care. Here, he pretty much admitted to copying the song and he got away with it. I guess Tim Rose (wrote the Banjo Song) and the Big 3 gave him a free pass. After all, the Banjo Songs lyrics were ripped off from Stephen Fosters 'Oh! Suzanna' so what comes around goes around I guess. Thats all I could find.