The primary difference between the two styles is that the kuro lolita look is necessarily monochromatic. That is, the kuro lolita dresses in all black.
The gothic lolita, on the other hand, wears more colors than just black (combinations of black and white, purples, etc). Additionally, it is my understanding that any type of lolita (gothic, sweet, punk) may be either shiro or kuro: dressing entirely in black or white. A gothic lolita, then, may be a kuro lolita as well.
Lolita fashion is always developing so this designation might not hold true forever.