八重咲きのヤマブキは、雄しべが花弁に変化し、雌しべは退化しているので、実が生りません。
Probably this black fruits are fruits of the kerria of a single flower.
As for the kerria of a double flower, a stamen changed to a petal, and a pistillate degenerated. So a fruit does not grow in a double flower.
