Wikipedia article about the album
According to Black Sabbath's guitarist and founding member Tony Iommi, the group's debut album was recorded in a single day on 16 October 1969. The session lasted twelve hours. Iommi said: "We just went in the studio and did it in a day, we played our live set and that was it. We actually thought a whole day was quite a long time, then off we went the next day to play for £20 in Switzerland." Aside from the bells, thunder and rain sound effects added to the beginning of the opening track, and the double-tracked guitar solos on "N.I.B." and "Sleeping Village", there were virtually no overdubs added to the album. Iommi recalls recording live: "We thought, 'We have two days to do it and one of the days is mixing.' So we played live. Ozzy (Osbourne) was singing at the same time, we just put him in a separate booth and off we went. We never had a second run of most of the stuff."
Iommi began recording the album with a white Fender Stratocaster, his guitar of choice at the time, but a malfunctioning pickup forced him to finish recording with a Gibson SG, a guitar he had recently purchased as a backup but had "never really played". The SG was a right-handed model which the left-handed Iommi played upside down. Soon after recording the album, he met a right-handed guitarist who was playing a left-handed SG upside down, and the two agreed to swap guitars; this is the SG that Iommi modified and later "put out to pasture" at the Hard Rock Cafe.
Apparently their original genre was "Heavy Blues Rock", later amended to the fathers of "Heavy Metal", now usually thought of as the great-great-great granddaddies of "Doom Metal".
I'm far more familiar with Paranoid (War Pigs, Iron Man, Planet Caravan, etc...) - so listening to this one is interesting to hear the early cues for the tunes from Paranoid. There's even a tune featuring harmonica...(The Wizard) and I forgot the funky bass intro to N.I.B
And Ozzy does a great job on vox - literally defined the metal vocalist?