Salt doesn't thicken blood - dehydration
can cause hemoconcentration.
When you eat salt, the amount of sodium and chloride in your cells & tissues increases. Anytime there is a shift in what the body recognizes as normal, or homeostasis (means everything is in balance), the body will compensate in some way to re-establish the balance. In this case the body retains water to get the sodium/chloride balance back to what it should be. At least that's the way I understand it. The blood pressure spike is caused in part by the extra effort the heart needs to pump blood thru blood vessels crowded by swelling or fluid retention. That and probably some mechanisms related to kidney
function that i am not smart enough to understand. That's it in a nutshell.