As you may know, many women are attracted to Black Men, but there are plenty who are not or have never entertained the idea. I'm going to assume she just has never entertained the idea, so you'll need to plant it in her mind, Inception-style. Conveniently watch a movie that features an attractive Black Male with at least one shirtless scene showcasing his physique. Make a compliment about his body by comparing his to yours as if yours is inferior. Nothing too depressing, but joke around like "He's got nothing on this body right here", clutching your gut or flexing your muscles in a very lame manner. Don't go overboard, otherwise, you'll just get the "Aww, but I love your body." All you want is to set a reference point.
After that, you can probably refer back to that scene during sex; but then again, it may be too soon to bring the freak out. What you may want to do is continue to lay the breadcrumb trail. Watch a comedy special that drops a line about Black Men having bigger cocks or take her to a basketball / football game and point out their height or physique. If you feel she's ready for it, you can bring out the big guns: a movie that features a BBC (Hall Pass, Sex Drive: Uncut, Any Given Sunday, etc.) or maybe even an IR porn. Now, if you choose an IR porn, your first choice should not be something hardcore like Cuckold Sessions or Cum-Eating Cuckolds; chances are, she will laugh her ass off and you will ******* all your chances of making this happen. Mainstream IR porn would be best. My personal favorite introduction would be "Little White Chicks Big Black Monster Dicks" since it has that campy, goofy vibe that women feel more comfortable with when it comes to porn. Yes, she will laugh, but now she "knows" what all the fuss was about with Black Men's packages.
By now, you should have already layed the foundation to her "knowing" what Black Men are presumably packing, so it may be safe to bring it up during sex. She might try to laugh it off, but you will then have to convince her that you are actually turned on by the thought of her taking a BBC like one in those movies. If you feel like she knows it genuinely turns you on, ask her to talk dirty to you about her fucking/sucking a BBC. Take it a step further and get her a realistic Black dildo. If you've gotten this far, the next step is to take it from fantasy to reality; and that, will have to depend on how good your relationship is at this point. Good luck.