My delicious green smoothies are typically about 35g carbs - and are 1.5 litres in size.
So portion size is key!
They typically contain:
50-100g Spinach or kale
A large courgette
2-3 inches Cucumber
1/4-1/3 of a punnet of Frozen Berries or cherries (berries are lower carb than cherries, so I use more of them)
1/4- 1/3 of a punnet of Frozen pineapple or apple
Some avocado or macadamia nuts
The perfect blend for a green smoothie is 60% mild tasting veg, 40% fruit.
Lob everything into the blender. Don't peel or remove the apple core, don't trim the courgette.
But DO take the skin off the avocado!

Add cold tap water until it comes half way up the fruit.
Blend til smooth.
Add extra water if necessary. I like a milkshake consistency, but it is up to you.
All ingredients and portion sizes are adjustable.
See the link in my sig for more ideas.