How 'bout flavoring with a bit of vanilla? Either drizzle in some vanilla extract, or toss a vanilla bean in the kefir or yogurt for a day or two in the fridge before making the smoothie.
I've been off my smoothie kick lately, been on a meat and eggs kick. But when I was doing low-carb smoothies, I would put in half an avocado and a quarter cup of berries. Berries and avos are much lower carb than other fruits - especially compared to, say, bananas and mangos or pineapple.