There are at least three or four different issues here:
1. 'Black Metal' as a genre vs. 'Black Metal' as a self-conscious artistic movement
2. Who was 'first' vs. who is most significant
3. Enduring contributions vs. ease of citation
4. Location, location, location
It should be fairly obvious that Venom was indeed among the first if not the first black metal band. However, their significance to the future development of black metal, especially as it emerged (in a self-conscious form) in the early 1990s is vastly overstated. It's pretty apparent at this point that Bathory and Hellhammer/Celtic Frost were not only much better bands, but vastly more influential as well. Venom's enduring legacy is largely accidental: they demonstrated that there was potential in dark, deliberately transgressive metal that made no compromises in the direction of an accessible listening experience. However, the real innovations and the basic lexicon of the emerging genre were provided by others: Bathory and Hellhammer/Celtic Frost most importantly, but Slayer, Possessed, Sepultura and Sodom as well. The continued emphasis on the importance of Venom is largely a product of accessibility (Venom albums have never been hard to find), intellectual shortcuts ("they have an album called Black Metal!") and simple geography (the most influential metal rags are based out of the US, Canada and the UK, and metal writers from these areas have always tended to overstate the significance of British and American acts).
