Beet likely contains metal chelators (like many vegs and fruits) that account for the improved athletic performance and lower blood pressure. But nitrate is not one.

Though raw beet juice may have far more therapeutic effect than heat-treated beet juice it also contains more toxins not destroyed by cooking. In the case of raw beet juice it may “increase your heart rate, cause fever or paralyze your vocal chords.” (http://health-care.eu/raw-beet-juice-a-great-cleanser-of-the-body where you can also see other benefits of raw beet juice). Fever is a sign your immune system is weakened by a high level of oxidative stress in this case due to the toxins of raw beet juice.