Place the blackberries, frozen banana slices, Greek yogurt, milk, sweetener, and vanilla extract into a high-speed blender container.

Secure the lid and blend on the highest setting for 45 to 60 seconds until the mixture is completely smooth and no large berry pieces remain.

Taste the smoothie and adjust sweetness if necessary by adding more honey, or adjust consistency by adding more milk.

Pour the contents into two glasses and serve immediately while cold.