Not an herbal treatment, but a mineral one.
Blood pressure is regulated internally by a balance of minerals, in particular: potassium, sodium and magnesium. We usually get a lot of sodium in our diets, not so with magnesium and potassium.
Magnesium isnt high in most foods, but almonds and buckwheat (as in pancakes) are good sources. As is broth – real broth made from your own bones – chicken, beef, pork, even fish; its the best source for all minerals, in the most bioavailable form.
All that said, I take a magnesium supplement at night (it also helps you to sleep).
Potassium in supplements is limited to 99 mg per pill, but the daily RDA is 3 g, meaning youd have to take 30 pills a day to get the RDA. Its high in bananas (about 400 mg), but higher still in potatoes (1200 mg or 1.2 g). If you dont want to eat a lot of potatoes, you can make “potato water.” Just slice a potato really thin (with the skins, as the nutrients are just below the skin level). Leave it to sit in a large glass of water overnight and in the morning drink the water and put the potato in the compost pile.