I like Blastoise, amazing design, good Pokemon, full of nostalgia and can help you on your journey, but it was the counter to Charizard not Venusaur.
Let's not forget that Venusaur is faster than Blastoise so it is going to hit first. Blastoise's special was 25 points lower than it is today. Grass moves automatically are special in gen 1, so Blastoise will be hit by crit hits and stab super-effective damage in the area where his defense are not quite as fortified. It could survive 1 razor leaf from Venusaur but not 2. It has to just one shot Venusaur on its turn, which even the strongest possible ice move Blizzard would not be enough to do. It would have to be very lucky and get a freeze hax. I was surprised to find that Venusaur's special was enough to withstand an assault even from Articuno when I did the damage calcs with RBY mechanics.