Dojo etiquette is going to vary from club to club. It’s always a good idea to ask what is okay and what is not when you are training somewhere new. At some clubs asking a black belt to roll is considered to be challenging them, usually not the greatest of ideas, while at others it is totally fine.
Here, I like to encourage people to ask higher higher belts to roll. You will learn more by rolling with someone that is better than you, than you will from someone that is around your own skill level. In fact, when I was training, the first person I looked to roll with was my instructor, and then I would work my way down the totem pole, always looking for the next best guy.
There are a few things you should keep in mind though. First off, as always, treat your partner with respect. The general rule of thumb is that a higher belt is going to treat you the same way that you treat them. So if you come after them hard, you should expect the same in return. Use your time wisely, ask them questions, see if they can help you with different scenarios.
Keep in mind that they are people too. They may be nursing an injury or just getting over one. I always tell people to feel free to ask me to roll, I may say no, but don’t take it personally. If I do say no it’s usually because I’m injured or not feeling 100%, on these days I prefer to roll with someone I know well, that is aware of the injury and I know I can trust them to be mindful of it.