My wife is the exact same way. She even refuses to watch videos I have made of us. She says she does not want to open the door to porn. I like many have not crossed the line of my wife taking a bbc.
My advice:
1. Dildos. Purchase a high quality dildo like Doc Johnsons Realistic 8" or Doc Johnsons Mr. Marcus. They are both the perfect size. If you could afford it, I would get both. They are both very realistic in looks and feel. The detail is incredible. It is the closest you can get to watching a real black cock sliding in and out of her sweet white pussy.
2. Hitachi Magic Wand. The magic wand is a vibrating wonder that can bring the most stubborn pussy to a quivering mess. I use this in combination with a dildo and drive my wife mad. The only problem is from keeping her from coming to quick.
3. Comliments. You need to complement her continually. Make her feel like she is beautiful and sexy. If she doesn't feel good about herself naked, she will never be open enough to play with dildos or bbc.
4. Confession. At some point you will have to broach the topic of hotwife/cuckold lifestyle. Tell her what you are feeling!
5. Fantasy. This can come first second or last, but at some point you want to connect the dildo to the concept of a real bbc in fantasy form. Once she has enjoyed the dildo a few times, if she is open to it, setup a fantasy scenario. Paint a picture in her mind of one of your fantasies. An easy one is a black massage therapist that goes to far. You can give her a long massage and talk as if you are the therapist. All the time narrating the scene, telling her what she is thinking and what he is thinking and what you are thinking. Try to get her thoughts connected with her orgasm. Give her physical and emotional pleasure in the fantasy of bbc and have her orgasm to these sensations and thoughts over and over and it is just a matter. At least that is what I am hoping for.
6. Time!
7. Time!
8. Time!