Every time I am faced with "Oh I am so ripe that I am going black" banana, I simply throw it in the freezer for later use. That mostly gets used in my pancake batter, cakes and/or breads. If you do not happen to have some in your freezer, simply slice the banana, place it on a plate and keep it in the freezer for 1-2 hours. Is freezing really required? No, not really. You can use it as it is and add ice cubes if you like it cold.
Now add all the ingredients to a blender (I added a date for additional sweetness and nutrition)
and blend until smooth. If too thick, you can either add some water to thin it or milk for extra creaminess. I add few splashes of raw milk until its creamy and flowy. If you do not have a powerful blender, you might require more milk for blending. Adjust seasoning according to your taste. My personal recommendation would be to avoid sour yogurt.