Grapes have a very subtle flavour that’s normally lost in a smoothie. However, they do add sweetness and have some remarkable health benefits.
Most people seem to use the seedless varieties, which I think is a mistake. Grape seeds are packed with nutrition – much more so than the skin and the pulp.
In fact a smoothie is the perfect way to add them to your diet. A decent blender will minimise any bits in the final mixture, so you won’t notice them.
They should always be washed first, to make sure they are clean of any residues. I often freeze them, and use instead of ice in recipes.