It doesn't necessarily have to be all blondes, or all brunettes, or only those with black hair. When I see a great artist, and Selena is a great example, it just knocks me out. There is nobody that is going to change my mind. Just something hits you. Now if you want to get real scientific, it has to do not just with the voice.
Good voices are a dime a dozen.
Of course, it should be a nice voice, a good voice. It doesn't have to be great. That comes with experience. There is a big difference. That's real critical in the Latin business.