Rule #1 don’t try and “push” her for the real thing. She’ll either be interested in fucking bbc or she won’t be, if she is happy days, if she isn’t, I wish you all the best suppressing your kinks and desires until seething resentment creeps in and you question the whole relationship.
That first pagraph is only me but talk to her about it and see what her thoughts are, anything other than a point blank “NO!” Is a good start. Then you buy big black dildos and role play during sex and then hopefully she’ll want the real thing.