It all started in 1967 when four working-class-lads from Aston, near Birmingham, England formed 'The Polka Tulk Blues Company':

Guitarist Tony Iommi (Anthony Frank Iommi, born 19 February 1948 in England),
Terry "Geezer" Butler (born 17 July 1949  England) on bass, Bill Ward on drums, born 5 May 1948 and
Ozzy Osbourne (born John Osbourne, 3 December 1948, England) on vocals together with Jimmy Phillips on rhythm guitar and saxophonist Acker.
After a while Phillips and Acker left and the band changed the name to the 'Earth Blues Company'(shortened to 'Earth'), playing a sort of jazz-blues fusion.

In early 1969 they played at clubs/college in the Midlands and at the 'Henry's Blueshouse' started by Jim Simpson, manager of the 'Bakerloo'(Dave 'Clem' Clempson on guitar!!) who also became their first manager.

Inspired by the 1930s Boris Karloff movie Black Sabbat they change their name after it and Terry Butler, who was interest in the occult and inspired by writers such as Dennis Wheatley and H.P.Lovecraft, wrote several dark songs including 'Wicked World' and 'Black Sabbath'.

After touring in England and Europe during 1969 they got a conctract in December that year with Fontana(Philips Records)  and released the single 'Evil Woman (Don't Play Your Games With Me)' in January 1970,  originally released by the Us band Crow.

In his last day at work at a factory, just before the recording of their first album in 1970, Toni Iommi lost the tips of the middle and ring fingers on his right hand in a piece of machinery.

He was told that he never going to be able to play guitar again but inspired by an album by Django Rienhardt(playing with two fingers on his fretboard hand) he decided to start trying to play again.

Iommi fashioned plastic thimble-like devices on his fingers and tuned his strings to lower notes which made it easier to play.

Perhaps the accident helped the band to get their unique deep riff sound.

The Album : Black Sabbath

Black Sabbath's eponymous debut album was released friday 13th February 1970 on the label Vertigo, recorded in two days in early 1970 at a cost of only 800 British pounds.

Due to Geezer Butler it was extended jams that was broken down into individual songs, it made no. 8 in the UK LP chart and reached the Top 40 in the US LP chart remaining in the charts over a year selling over a million copies.

The Songs on Black Sabbath: 

'Black Sabbath' [6:18]
I still remember when I heard the opening track for the first time, back in 1970, the beginning when the sound of rain fades in, at a distant a tolling church bell, nearby thunder and then suddenly a three-note heavy distorted guitar riff repeating with pounding drums matching up with the tolling church bell,
1 minute and 23 seconds into the track Ozzy's haunting vocals starts with the words:
 
  'What is this that stands before me?
  Figure in black which points at me.
  Turn around quick, and start to run.
  Find out I'm the chosen one.
  Oh nooo!'

...still the three-note repeating guitar riff with the pounding drums, 2:18 Geezer Butler thunder bass enters the song
..what a beginning!!..it ends with a uptempo and a great guitar solo.

'The Wizard' [04:22]
A sort of blues number, starts with a basic harmonica lick by Ozzy then guitars, bass, and drums join in following the harmonica. 00:41 a short guitarsolo then the song is driven by great guitar riffs together with the harmonica and singing mixed with short guitar and harmonica solos.

'Behind The Wall of Sleep' [4:18]
Begins with an nice 30 seconds instrumental intro ('WASP'). The song by Ozzie is build as a stereo effect(two-track technique, left and right speaker), the song fades out with Bill Ward's drum beat and ends with a 30 second 'bass solo'('Bassically') by Geezer Butler as a bridge to the next song.

'N.I.B.' [5:23]
which opens with an excellent heavy distorted bass riff by Geezer Butler with the first four notes from the Cream's 'Sunshine Of Your Love'.

This is along with 'Black Sabbath' the best track on the album with superb guitar solos from Iommi and song from Ozzy, a classic track.

The title is about Bill Wards "nibbly" beard but the song is about the Devil who falls in love, cute.

'Evil Woman' [3:24]
Sabbath's first single a poprock cover originally released by the Us band Crow. Nice crunchy bassline by Geezer.

'Sleeping Village' [3:45]
The beginning of the song has a acoustic part(53 seconds long),  sounding a bit of one of Ennio Morricone's Western tunes it soon segues into some crunchy guitar riffs and a twin guitar solo(left and right speaker) by Iommi.
The song is for me a filler or as an intro to the next song

'The Warning' [10:27] 
a cover of a Aynsley Dunbar's Retaliation song(released 1968 as a 7").
The Warning opens (again) with a crunchy bass riff, strong song/voice by Ozzy. The first part of the song, 3 minutes and 20 seconds is the best then it's six minutes of Iommi's astounding guitar solos(great stuff!!), 9:33 into the song it's time for the final part: back with  the crunchy bass riff and Ozzy's lyrics:

'Now the whole wide world is movin'
'cause there's iron in my heart
I just can't keep from cryin'
'cause you say we've got to part
Sorrow grips my voice as I stand here all alone
And watch you slowly take away
A love I've never known
I was born without you, baby
But my feelings were a little bit too strong
Just a little bit to strong'

Epilogue

By the way, the front cover of the album shows a ghost–like woman but it was the upside down cross on the gatefold of the album that would associate the band with Satanism.

The cross was an idea by the record company and was not supported by the band so then it must be the record company that worship Satan or?