wasg88 offers good suggestions, I'll add a few more.
Wives usually prefer something a little more than the regular "hump and fuck" interracial movies. Story lines, exotic settings and romance seem to have a higher priority for women.
You may want to try renting or purchasing a Sean Michaels video so that your wife can see a striking Black Bull in action - I will suggest "The Dinner Party" & "The Dinner Party II: The Buffet" as good Sean Michaels videos to start with.
Both of these movies are full length feature movies with a real plot and quality production values. Sean has dynamic appearances in both of these movies, but is not the main star.
"Janine's Been Blackmaled", "Passion In Venice", "Stiletto", and "Once You Go Black 3" are a few of the better videos with Sean Michaels.
I will suggest that the next time you and your wife watch an interracial movie have a "little surprise" for her in the way of the following, letting her know that this is a realistic size. If her pussy gets hot and wet just looking at them - wait 'til she gets one in her hands!
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If your wife is as "prissy" as mine is (and I believe NO ONE has a wife as prissy as I do) try the following "surprise" (also marketed as "Smooth Cock and Balls"). I was slamming my wife with this one "ALL the WAY" within minutes - it goes in smooth & easy and will stretch her out!!!
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There are a lot of interracial videos out there, many are geared towards guys or the "experienced woman", such as Multi-Orgasmic Mary's video series.
Take it easy and take it slow. You want to progress to the point that you and your wife are going to the dance clubs frequented by Black Bulls.
Good luck and keep us posted on your progress - we're all here to learn from each other.