Read Dr. Timothy McCall's response:
Before working with students with high blood pressure, known medically as hypertension, encourage them to be evaluated by a doctor (if they haven't been already). That way you'll know that serious medical conditions that can cause hypertension—or those that can result from it—have been ruled out. You'll also know that the pressure is not so high that some yoga practices, such as Sirsasana (Headstand) or Kapalabhati Pranayama (Skull Shining Breath), would be contraindicated.
From a yogic perspective, practices that balance and calm the autonomic nervous system (ANS) are often useful in hypertension. Many people with high blood pressure are stressed, and the sympathetic branch (the so-called "fight or flight" system) of the ANS tends to dominate over the parasympathetic branch (sometimes called the "rest and digest" system). That means relaxing yoga practices are key. If I had to choose one pose for hypertension, it would be Savasana (Corpse Pose). In fact, there are medical studies that have found that this pose all by itself it can lower blood pressure significantly.
But yoga therapy is never about just one pose. Many stressed people wouldn't be able to do a decent Savasana unless they'd burned off some steam with a more vigorous practice first. So a balanced practice, appropriate to the student's level, followed by Savasana and, if the student is up for it, some relaxing pranayama and meditation, would be ideal.
As always, a little yoga every day beats a long session once a week.