There are "public" spaces that may be "too" public for a white couple looking for a black man to fuck the wife while the husband watches. For example, I would never take my wife to a mall to pick-up another man as one does not routinely go to a mall for sex. A mall also has a diverse spread of people there, including minors. Yes, a couple might just get lucky, but the odds are certainly running against them.
When my wife was "in heat" for Black cock, we would go to a venue which had a sexual element to it and one where we would expect there would be men, especially Black men, looking for sex. An example of a place that usually worked for us was a porno theater, in an area with a large Black population. Men, including black men, go to a porno theater because they want to get off. They do not go there to have a hot game of "Scrabble". Other places would include nudie bars, strip joints, and go-go bars. And if a white couple walks into such an establishment, the odds are that they are also looking to get off, in one way or another.
It is important that the woman (wife or girlfriend) dresses to "attract". It can be subtle, such as a sheer, tight top with no bra, with her stiff nipples clearly visible. Or, as my wife always did, she could be dressed like a cock-crazy whore: short, tight dress with a plunging bodice, nylons, heels, no bra or panties. A hot wife marker, such as the QOS, would be icing on the cake. (Or pussy) The couple should take seats where there is clear access to the wife from the aisle.
Most men are reserved and will act like gentlemen in the theater. They will initially allow the couple time and space (if nothing else, the eyes have to adjust for the dim lighting in the theater). The husband/boyfriend could send out subtle hints like caressing the wife's knee or kissing her on the lips. Eventually, men will gather around the couple. The most aggressive alpha-male will take the seat next to the wife and begin a friendly conversation with both husband and wife. (How are you all doing tonight? Is it still humid outside?) Usually, the conversation will gradually take on a more sexual nature. The alpha-male may break the sexual tension by commenting to the couple "You have a very pretty wife; her legs (teats, etc.) are to die for. Are you folks looking for some action (fun)?" The cuckold husband should answer such as: "My wife is horny and is "in heat". She wants to be fucked really good by a "REAL" man like you. And I want to watch. Care to help us out"? The wife shows her desire by revealing her teats and/or pulling up the hem on her dress and spreading her legs. At this point in time, nature should take its course. Details as to exactly where the fucking/cuckolding will take place will be decided in the heat of passion.
Footnote: If the decision has been made by the couple that the slut will be gangbanged in the theater, take seats in the very first row so that multiple men could have access to the wife's female charms.