To a high-speed blender or small blender, add banana, blueberries, spinach, protein powder, dairy-free milk (starting with the lesser amount), and seeds (optional) and blend on high until creamy and smooth, scraping down sides as needed.
Taste and adjust flavor as needed, adding more banana for sweetness or protein powder for peanut butter flavor. Add more dairy-free milk as needed to thin.
Serve and enjoy immediately. Store leftovers covered in the refrigerator up to 24 hours, though best when fresh. You can also freeze leftovers in an ice cube tray and add to future smoothies.
*Nutrition information is a rough estimate calculated with unfortified almond milk (the lesser amount) and without optional ingredients.