There is a key difference between blenders and food processors, which is why you should have both in your kitchen: the differences lie in the motors and the blades. For blenders the motor is powerful, but the blades are quite dull, while for the food processors it’s the other way around: the motor is weaker, but the blades are extra sharp.
These key differences are why the two are not interchangeable when it comes to what you can prepare with them. The blender is useful for anything that needs to have an even consistency and have a super smooth texture, such as milkshakes, smoothies, and cream soups (you can always not blend them for longer if you want a chunkier cream soup).
While the food processor is needed for anything that has fewer liquids and hard foods, like nuts, garlic, and raw onions, and like pesto, which is known for its chunky texture.