There is a perfectly clear mechanism by which increased salt can increases BP. Sodium (and potassium) have to be at the right concentration in the body for it to work properly (nerves cant' be activated if the concentrations are wrong) So, if there is too much sodium in the bloodstream, the body increases fluid to dilute it. If there is more fluid the heart has to work harder to push the increased volume around the circulation.
As has been said most salt comes from processed foods. The manufacturers add it for increased palatability and for preservation. People who eat a 'Western' dietary pattern with lots of processed foods therefore take in a lot of dietary sodium in them
In the UK and other countries. governments have gradually compelled food manufacturers to reduce salt in their foods. It has been effective. There is now quite a bit less salt in these foods. This has been accompanied by an overall lowering of blood pressure in the population and there has been a drop in heart attacks and particularly strokes in these countries that can be at least partly attributed to this measure.
( Countries like China where a lot of salt is actually added during cooking .need different tactics)
Although it's working on a population basis, everyone is an individual. If you eat mainly non processed foods and don't add a lot of salt or salty flavourings like soy sauce then you are probably don't need to reduce your salt intake. if you have high blood pressure, and add a lot of salt in cooking or at table then one method that may be of help is to take in less salt.
Yesterday I watched these two presentations from the recent EASD meeting (and some of my answer is based on what I saw)
The first argues the case for salt reduction in the population and for people with diabetes who have high blood pressure. (you can see an outline of all the evidence)
The other argues against the point particularly at the individual level.( I think very much a devils advocate argument when certain drugs like ACEs are also involved ie food for thought for clinicians who make blanket statements)
http://www.easdvirtualmeeting.org/resources/19070
( The Prof arguing for the reduction of salt is specialist in hypertension and a UK government advisor. I'm sure many people will be pleased that he started by saying why are we disusing this? We've done it and it has worked. What we should be debating is how to reduce sugar at a population level He headed up the Action on Salt lobby and has now turned to sugar
http://www.actiononsalt.org.uk/actiononsugar/How to eat less sugar /133916.html .)