Put the oats in the blender jar first and add the milk. Let them sit for 2–3 minutes while you gather everything else; this softens the oats so they blend smoothly.
Add the yogurt, banana, blueberries and cinnamon. Putting the frozen fruit on top keeps it from jamming the blades.
Blend on low for 10 seconds, then on high for 45–60 seconds until completely smooth and no flecks of oat remain.
Taste. Add the honey only if your banana was not very ripe, and blend a few seconds more. Thin with a splash of milk if it is too thick to pour.
Pour into two glasses and drink right away.