How can potassium rich foods reduce blood pressure?
potassium is really the main guy in charge of everything that happens involving action potentials which in turn govern homeostasis such as blood pressure. 

k+ lowers blood pressure due to making it harder to get an action potential going, which in the macro scale causes the heart to pump less.

too much k+ kills the person as it stops the pumping of blood (lethal injections are just lots and lots of k+ along with some pain killing drugs)