Interesting double blinded placebo controlled article in  Hypertension 2015 Feb 65:320, where 64 patients were given 250 ml of beet root juice for 4 weeks, and all of them saw roughly a 7mm reduction in systolic (top) and a 2.5mm reduction in diastolic (bottom) numbers.

Inorganic nitrates in beet root juice when consumed are converted to the vasodilating nitric oxide which is responsible for lowering the blood pressure. Diabetics however need to be aware of the high sugar content in all juices