Blackberries and boysenberries do have quite a bit in common, though there are still plenty of differences that separate them as being completely different as a whole.
Although it is true that both fruits have seeds, boysenberries have much smaller ones than blackberries. Another difference between these two fruits is that blackberries have a black color and boysenberries are more maroon. As far as the shape of these fruits is concerned, they differ quite a bit with blackberries having a pine-cone shape while boysenberries are rounder.
While many people mistake these fruits as being essentially the same, the truth is that there are quite a few differences.