British phenom Adele and rapper Eminem are two of the music industry’s best-selling artists, having moved millions of units and digital downloads this year. Aside from their commercial success, there’s also something even more unique about both: they are part of an exclusive cadre of white artists who have hit it big with musical styles that make them popular amongst both mainstream and black audiences.

That is no easy feat, as the road to the Billboard charts is littered with so-called “blue-eyed soul” singers whose successes can be ephemeral. Though it may be hard to remember now, there was once a time when the likes of mainstream artists such as Madonna and New Kids on the Block were once mistaken for black singers, or wholeheartedly embraced a soul-infused sound. In the intervening years, numerous other artists ascended to stellar heights singing rhythm and blues (R&B) or rap music.

Over the last few decades, however, a once lengthy list populated by names like Teena Marie, Lisa Stansfield, and Dusty Springfield has been culled to a much smaller number. Those who remain – such as U.K. divas Joss Stone and Amy Winehouse – have seen endless amounts of ink spilled about their relevance and staying power within a genre whose singers are overwhelmingly black.

Click here to view a slideshow of theGrio’s favorite blue-eyed soul artists

So why exactly do white soul singers catch fire in ways some black artists don’t? Theories abound, but one major factor may be society’s deeply ingrained beliefs about how white artists should sound.

“It’s fair to say that blue-eyed soul artists have always flirted close with being novelty acts, not because that’s their intention but because of society’s rigid, racial assumptions…that turns any white person who can credibly sing [into] a black aesthetic,” Oliver Wang, a sociology professor at California State University-Long Beach, told TheGrio.com.

Wang, the author of Classic Material: The Hip-Hop Album Guide, says that the preponderance of talented black singers can turn comparable white artists into an irresistible curiosity to mainstream audiences. Therefore, blue-eyed soul singers “seem extra special whereas, if a black artist sounded identical, that’s not seen as necessarily remarkable.”In other words, black singers may be victims of their own talents. Audiences are fickle, and will gravitate to the first thing that looks unusual.

“Thanks to shows like American Idol, society is starting to get numb to the black girl who can sing her ass off,” quipped Steve “Funkworm” Butler, a Chicago-based independent music producer and blogger. “Most believe that they can walk into any black church on Sunday morning, close their eyes, reach into the choir stands, and pull out a exceptional singer. And this is partially true.”

For that reason, ambient resentment toward blue-eyed soul artists has festered for years – most notably when George Michael (who has since gone pop) scored a controversial win in the R&B/Soul category of the American Music Awards back in 1989. That led to misguided assertions that white singers might be ‘taking over’ a medium pioneered by blacks for blacks.

“The larger issue is just that African Americans have had to be more protective of their cultural capital just because it’s the only capital that they had,” explains said Joseph G. Schloss, a professor at Baruch College in New York and author of a book on hip-hop culture in New York. “So it’s not so much about the music as it is about preserving resources for the community.”

Virtually since R&B was in its infancy, white singers have had a place in the genre in ways large and small. For some, the flameout comes quicker than others: several Caucasian soul singers have suffered a painfully short half-life. In many instances, these artists cross over into even more successful pop careers.

Fairly or not, the permutation from soul to pop has led to some accusations that white singers cynically manipulate black audiences to achieve mainstream career advancement. Industry observers acknowledge an element of opportunism, but add that it’s also contingent on how faithful the individual singer is to his or her black audience.

“It’s more about the artist working to maintain a relationship with the African American community as much as it is about their specific musical choices; and each can influence the other,” Schloss said. “I think Teena Marie would be a perfect example of that.”

The majority of white soul singers and rappers (Eminem being the among the most prominent examples) work hard to maintain the loyalty of black audiences. Yet there’s little doubt that some blacks view white artists with suspicion. Here’s where territoriality rears its head: because soul has historically been black, some may resent the encroachment of white artists on this turf.

But what determines whether a blue-eyed soul singer explodes like Robin Thicke, or fizzles quicker than neo-soul crooner Remy Shand? Timing may be everything, experts say: some argue that only one or two white R&B artists at a time can hold a grip on the public’s imagination successfully.

Evidence strongly substantiates this theory. After all, the prominence of Stone, Winehouse and Adele materialized in successive waves. Once one career cooled, the other’s seemingly gained momentum.

This differs from the paradigm exhibited by black R&B artists, which often sees multiple singers occupying the limelight at the same time.

“The problem with being treated as a novelty of sorts is that there’s usually only room for one at any given time, thus limiting their opportunities,” said CSU’s Wang. “In contrast, you can have Kelly Rowland, Beyonce and Rihanna all chasing after the same market simultaneously.”

For that reason, charges of white soul singers “taking over” seem overblown at best. Particularly because according to industry watchers, the politics of the music business and entrenched cultural preferences can play a decisive role in the career of white soul artists.

“It’s still very hard to break a white singer into the urban market,” Butler argues. “These artists, for the most part, still need to be cuddled by successful black producers or collaborate with successful black artists. That cosign makes it a little easier for them to get accepted by black audiences.”