The black star dragon balls were separate from the regular ones and were kinda just added for no real reason other then plot since they never appeared on the radar in dragon ball z
they were created by unfused kami and King Piccolo.
The Black Star Dragon Balls are apparently the same size as the Earth Dragon Balls approx. 7.5 cm. It can summon Black Star Shenron that can grant any wishes, including killing people, which the original Shenron cannot. Unfortunately, to counter such unrestricted powers, these artifacts have a lethal side effect. After the wish has been granted, the Dragon Balls spread across the galaxy. If all seven balls have not been re-collected and returned to the planet on which the wish was granted in one year, the planet will explode. Also, after the wish has been granted, these Dragon Balls do not turn into stone, so if someone does collect all seven, they can wish again right away.