A slow heart rate can absolutely cause a high blood pressure or at least be a big contributing factor.. if the heart is beating slow, that means there is a greater amount of time for the atria and ventricles to fill; so they're pumping more blood volume through the heart,through the system per each beat.. leads to a greater cardiac stretch therefore raising the pressure and systemic blood pressure as well.
Sometimes high blood pressure and a rapid heart rate are concomitant conditions indicating the heart is under stress...a slow heart rate will not cause hypertension.