I've got to disagree there - the first three Sabbath albums have a very clearly defined mix to my ears. It's a very live, hard stereo-panned sound and I've always found it easy to pick out Geezer's basslines.
Vol 4 is a murky mess though.
I always felt Geezer sounded a lot more discipline on the Dio material, but then so did the band as a whole. Less improv.