How important is this to you? Would you truly be happy with a woman that would never cuckold you? If this urge is strong beyond your control and you find she's not willing to live it with you, then maybe you need to find someone else. If being a cuckold isn't that strong of an urge for you, then take is super slow and just hint around, and if she doesn't seem into it, then it won't bother you that much. On the other hand, if imagining a black penis sliding in and out of her makes you hard as can be, then i think it's important that you have a woman willing to make that happen.
I really think the most important thing here is that you are certain of who you are and then find out if she is a match for you.