Maybe someone thought about it mechanistically in the sense that more sodium in our blood also means more water (to keep both electrolyte balance and dilution in acceptable ranges to sustain life). More water means more blood volume or pressure if volume is limited.

Someone then probably tried to prove this by checking whether dietary salt intake leads to hypertension. However, medical and nutritional science are very limited in the ways they can conduct research. So they probably did an epidemiological study and saw that populations with hypertension also had a high dietary salt intake. Proof! Except... well, no.

Our kidneys are perfectly capable of balancing this out as long as we don't ask too much of them. More salt in your food than you need? More sodium in your pee. Less salt in your food than you need? Well, you're screwed.

Edit: Sorry for being unable to provide a direct answer to your question, but I felt compelled to reply.

Edit 2: After a search in Google Scholar, the earliest study I could find is from 1950 [0]. They studied the link between salt and hypertension in rats by replacing their drinking water with 2% salt solutions and measuring their blood pressure. It seems to me that all non-epidemiological studies are useless extremes... With no "clean" water (all water intake was hypertonic) the kidneys can't really dispose of much.

[0]: http://sci-hub.tw/https://journals.sagepub.com/doi/abs/10.31...

Edit 3: The actual paper recognises in the discussion (with citations) that it is impossible to cause hypertension in rats without artificially controlling all their water intake. Wow. Even if all water is salted, as long as it's iso- or hypotonic there is no hypertension.

Just an anecdote, from my experience of water-only intermittent fasting for 5 days. First time I did it, from around day 3, I will start feeling dizzy, if I suddenly standup from either lying down or sitting in a relaxed posture. Later I read that it was caused because of low blood pressure due to no salt intake. The next time I did intermittent fasting, I took some salt with my water, and the dizziness problem was not there.

I read long ago that a famous american cardiologist simply asserted that high salt intake leads to high blood pressure, and that it has been known as a fact since then.

Regrettably I can't find the source.