I put beetroot powder in smoothies. Powdered beetroot thickens liquids considerably and the taste is kind of overpowering with an earthy note* so start with 1 teasp[oon per smoothie and see how your smoothie turns out. Beetroot is a natural nitric oxide booster and you should consume it right before a workout because it helps recovery. Otherwise, it's best taken in the evening because that's when we make less nitric oxide.
Nice thing about smoothies is that there are no rules. Anything can be put into them. I am not into smoothies but take a beet powder drink daily. As stated, it helps with endurance when taken before physical activity and it naturally lowers blood pressure.