Well, isn't practically all Venom an assault of blues-punk riffage in the vein of Motorhead and Tank played too fast, verse-chorus oriented, mostly rather upbeat even though brutal and always using the traditional NWOBHM style phrases ? Bathory seems to hint at building an evil ambience, "softer" you might say, through the way the guitars drone on and the beat remains constant, chorus being understated, already on the first album. But then take something like "Total Destruction" or "Sadist" from "The Return" and the music is definitely closer to something like Discharge or early Cryptic Slaughter than Venom; the riffs become strobing patterns, not bluesy at all, and I don't think its very conventional to structure the song in that way, there is no "chorus riff" at all. This minimalism and primitivism, oriented away from rock music, is what makes the black metal experience for me.
I was also introduced to, and liked, both contemporaneously, but I might be biased from having been familiar with newer black metal already. But I give credit to Hellhammer being the first black metal band clearly distinct from heavy metal; all that stuff they released in '83-'84 is end of discussion, though they admit BIG Venom influence, unlike Bathory.