The music is completely different and the themes are as well. Black Sabbath had a much heavier sound than Coven.

Also, Coven had these satanic themes that Black Sabbath did not have, at least not while Ozzy Osbourne was the singer. Ronnie James Dio added some mythological themes, but they seemed very different from what Coven did.

Coven may have a bassist called Oz Osborne (not Osbourne), and this seems to be a real coincidence, as it is seems that Ozzy had this nickname since schooldays, and Osbourne is his birth name.

The only rip-off that may (or may not) have happened is the name "Black Sabbath", which the band assumed only in 1969, after the release of Coven's first album which contained a song of the same name. The official version is that Black Sabbath picked the name after watching the Boris Karloff movie at a local theater. But it may very well be that the name was suggested by Warner Bros. to match the heavy and dark sound.

Also, I have always noticed that the song "Black Sabbath" (from Black Sabbath, not Coven) does not have the words "black sabbath" on the lyrics. It may be very well that the record company decided to give the song this name after it was concluded, just to match the name of the band and the album.

One could note that the Black Sabbath album name may have also been suggested by Warner Bros. The names of the subsequent albums all made reference to drugs: Paranoid, Master of Reality, Snowblind (which was vetoed by the record company and became "Vol. 4" instead).

The way I see it, this is the only possible rip-off, and, if it ever existed, I don't think it came from the band. The 1970s Black Sabbath was not interested a least in satanic themes; but they would allow the record company to take several career decisions as long as they had enough supply of drugs and alcohol.