That actually doesn't look like fur turning gray, as much as she is shedding! If you lightly tug along that perimeter, you'll probably be able to get some fur off -- From my experience, this is what black-fur-rabbits look like as they are shedding. It makes grooming easy because I can see exactly where I need to concentrate my grooming focus.
If you can see, her shorter hair (on the right) is completely black, but the "gray" hair is very long in comparison. Once that fur comes out, new fur will fill in, giving the all-black appearance again.
It seems like this may be your first big molt with her, so check her fur every day to see if there are any large places you can easily ease out fur with a brush or your hands -- this will prevent her from eating too much fur while grooming, which can block up her stomach (very dangerous). You may not be able to get fur every day, but checking every day is important.
https://www.instagram.com/p/BgJD60YB9RM/
If you look here ^ you'll see a similar pattern of gray looking fur, but it's from shedding.