Venom is what started it all. They had the satanic imagery(though they weren't serious) & they inspired Hellhammer/Celtic Frost, Mercyful Fate, Bathory, Possessed, Bulldozer & Sodom. Bathory took it to the next level & ended up creating the sound that the later BM bands would adopt. Hellhammer/Celtic Frost would also end up being influential to both BM & DM.
Honestly, just a technical point: Mercyful Fate predates Venom by a bit, or if you don't count the older demos, is at least contemporaneous with them. They don't really sound like Venom. If anything, early Mercyful Fate occasionally resembles Angel Witch and Judas Priest to me. I've never heard anywhere that they were influenced by Venom, but if this indeed so, enlighten me.
Venom did really sound like Motorhead though, in every sense of the word. And, the recognizable Venom influence on Hellhammer starts to fade by the Satanic Rites demo, though I'll admit, is present. To deny the fact that Venom influenced Hellhammer in some capacity or another is absurd, since the band itself claims to have been influenced by them. Interesting anecdote though: upon playing "Horus/Aggressor" to a friend of mine not familiar with metal past the 70s, he remarked upon it sounding like a perfect cross between Motorhead and Black Sabbath. Perhaps Venom was more influential in spirit than in actual music.
Quorthon consistently claims he hadn't heard Venom, and it's conceivable that having listened to GBH and Motorhead, as he claims, he could have come to a similar conclusion musically, seeing as Venom was in essence Motorhead+punk+Satanic imagery.
Haha, my apologies for digging this up a month later.