Why are blackberries and raspberries not true botanical berries?

In botany, a true berry is defined as a simple, fleshy fruit derived from the single ovary of an individual flower.12

Blackberries and raspberries are not true botanical berries because they are aggregate fruits. Instead of developing from a single ovary, these fruits form from a single flower that contains many separate pistils. Each of these individual pistils develops into a tiny, individual fruitlet known as a druplet—a small drupe containing a single seed. The final fruit that we eat is actually a cluster or collection of these numerous drupelets.1324