Venom may have been among the first to play black metal, but they don't even come close to being as influential and unique as Bathory. If not for Quorthon's genius, inventiveness, and innovation with the dark art of black metal music in his early years, I would argue that there'd be no such thing today as "black metal" or even "death metal". Venom were only playing thrash / speed metal in a minor key with mystical lyrics about Satan and other dark things, while Bathory was actually creating a new style of metal entirely. Quorthon began to add elements from thrash, speed, prog metal, industrial music, and even classical music into his songwriting. This experimentation with various styles led to one unique sound that has been copied countless times by black metal bands today. The earliest example I can think of is Darkthrone's Transilvanian Hunger. This album is often cited as the one that most closely resembles the sound on Bathory's first three albums, and it would not have existed if Quorthon had not experimented with various styles on his own earlier works.