Venom did not invent black metal. They were one of the very first bands that played extreme metal music. They inspired too many bands and musicians. Some of the very early thrash bands (Metallica, Overkill) have said that they Venom was a major influence.
Norway has nothing to do with "starting black metal". The bands from Norway were the "second wave" of black metal, and that was early 90's.
This is a bit confusing. On the one hand, you say that Norway didn't invent it because those bands were the second wave of BM, which is true. On the other hand, you also don't give credit to Venom, although they played a major part in the First Wave of Black Metal.
Because this First Wave consisted basically of bands like Venom, Bathory, Hellhammer/Celtic Frost, Sodom, Destruction and so on, whose music at that time was called Black Metal.
And I think that's exactly the important point here. Those First Wave-bands didn't play music you'd call Black Metal nowadays. This style was probably indeed developed in Norway in the early 90's (and called Death Meal in the beginning, e.g. by Euronymous, to set it apart from the 80's Black Metal). But the term and this type of poorly produced Heavy/Thrash with weird vocals and satanic vocals first called Black Metal originated in the 80's. Venom weren't the sole inventors, but they were one of the most important bands to shape it.
And since we're talking videos already, one of the best (and probably most entertaining) takes on documenting Black Metal's history was performed by one of Norway's most kvlt BM-vets, Darkthrone's Fenriz:
edit: after reviewing Fenriz and thinking about it, I think the biggest credit should go to Quorthon. But Venom were pretty important as well.
