The first thing you need to know is that a true Satanist will tell you that any group that puts Satanic imagery on their album covers or in the music is bogus. Every single one.
The other thing you need to know is that a true Satanist will also tell you that all music is Satanic, every single note.
Consequently, an article on Satanic rock is a tricky proposition insofar as none of it is real; explicitly Satanic rock like Venom is really no more Satanic than The Carpenters. And the Carpenters are just as Satanic as Venom.
However, there is a history to the appearance of Satanic imagery and references in rock music. So consider this an attempt to organize that history a little.
Part I: The Blues
The Blues has always been the Devil's music. Music that glorified drinking, womanizing, gambling, dope, violence, and depravity, blues was an easy target for ministers and pastors of the South, who countered with sermons forbidding the congregation to listen to it. Many god-fearing churchgoers heeded this message, establishing Gospel as the safer alternative. Even some bluesmen were convinced; legendary blues picker Gary Davis usually refused to play blues after he was ordained as a reverend in 1937. Ultimately, he relented just before his death and recorded a historic session of blues (secular and gospel) in 1971. He died soon after.
One of the most pervasive legends surrounding the blues is that of legendary delta guitarist Robert Johnson, often considered the first bluesman in the chain that ultimately pointed towards the development of rock 'n' roll. Johnson was an acoustic player of the 1930's who died under mysterious circumstances in 1938.
The legend went that Johnson, not blessed with guitar talent when he first began playing professionally, yearned for overnight success that would put him in league with the other guitarists on the circuit. One night, he heard a voice that told him to visit the crossroads by Dockery's plantation at midnight. There, he was met by a large black man who apparently was the devil in disguise. The big man took the guitar from Johnson, tuned it, and returned it to him.
Johnson's improvement on his instrument was swift and amazing (although historically, it took him about a year to become great). He earned the instant recognition of big name guitarists like Son House, who championed his cause. However, Johnson was tormented in his dreams by visions of the devil, and hellhounds on his trail. In his waking hours, Johnson played the role of bluesman hero, chasing women, drinking, behaving arrogantly. In 1938, during a show, he was poisoned (possibly by a jealous husband of a woman he had been putting moves on). The poison had him foaming at the mouth and talking babble, he died within days. His last words were "I pray that my redeemer will come and take me from my grave."