That's the genius of it!
No, but really, while it can be easily argued that most forms of popular music have their roots in traditional types of black music, it doesn't make it of 'black origin' as defined by the MOBOs.
The spirit of the awards (I think anyway) is to promote genres of music which are directly associated with or come from black communities, i.e. hip-hop, soul etc. So while, say rock music, undoubtedly came directly from Blues music it was and was developed in its early days by black musicians it exists in its current form almost entirely due to musicians coming from a white heritage. So a rock act doesn't qualify for a MOBO award, regardless of the colour of the artist(s) skin. Similarly with Country music.
Conversely, something like hip-hop or 60s-style girl groups exist directly because of black talent, and so a hip-hop act would qualify for a MOBO even if they were white (or Asian, or Polynesian etc).