Salt doesn't cause vasoconstriction (constriction of blood vessels). That's not really in dispute. It's whether increased salt intake leads to increase blood pressure. The theory behind this is that increases salt in the vascular spaces will draw in more fluid volume through osmotic pressure, which results in higher blood pressure.

The problem is there's a lot more going on in the body than basic hydrostatic forces and in general the body is pretty good at maintaining homeostasis in the face of external perturbation. As one, very relevant example, the kidneys are constantly excreting both salt and fluid in opposition to various changes in blood pressure and chemistry.

We know there are patients who are sensitive to salt in their diets. Particularly those with decreased kidney function. This and some other studies are asking the question whether increased salt intake in otherwise healthy individuals leads to an increase in blood pressure. So far the answer seems to be a resounding ...maybe.