It's an interesting question as I recently saw a black and white roborovski up for adoption and had to look twice. I had never heard of robos coming in that colouring but apparently there is a "mottled" or piebald colouring of black and white. I've not heard of an all black one though. A photo would help.
According to Wikipedia there is a recessive all black bred in Finland - quite rare by the sound of it.
Roborovski dwarf hamster - Wikipedia
Do you know how old she is? If she is very young she could be a tiny russian dwarf but from the above link it seems there are some black robos!
Have a look at the back legs. Robos are quite distinctive in that way. They have longer legs than most species of hamster and it's more noticeable with the back legs.
Make sure there is only soft substrate to land on near the wheel if she's flying out!