Walnuts, like any nut, is a grand addition to any smoothie.  You can blend the walnuts or crush them and sprinkle them on top.  The addition of walnuts makes the smoothie creamier while appending healthy fats and protein.  It's definitely leveling up your game. 
 
The nut taste is mild, earthy, and a little tangy, but somehow it overpowers other tastes making it a good blending nut.   For example, mixing it with blueberries will smooth out the acid taste.  It's also interesting to use it with banana and mango. 
 
The two most popular classes of walnuts are the Persian or English walnut and the black walnut. The English walnut originated in Iran, and the black walnut (J. nigra) is native to eastern North America.
 
In Israel, walnuts grow in the Galilee and the Golan.