From the man himself:
"When we recorded Black Sabbath, I had a 70-watt Laney guitar amp and a Park 4x12 cabinet with only three speakers in itand two of them were wrecked! Thats how I got that really distorted sound. Actually, I hated the tone of that record at the time, but Ive gotten used to it now. Its nostalgic. I didnt have any alternative; I couldnt afford to buy new speakers. We had only two days to record, so we just plugged in and performed our live set in the studio. We were allowed one take for each song and stopped only if someone made a horrible mistake. It was out of our hands. No time to dial in the perfect bass tone."
From experience the key to that sound is a P bass played hard over the neck, similar to early Entwistle. Get those strings bouncing off the neck.