Setting aside any possible physiological difference in the actual vocal cords, black people have a different phenotype than, say, white people, including:
* a different shape nose with a lower bridge that, combined with markedly higher cheekbones, allows for a larger nasal cavity
* a longer and wider set jaw bone that make their mouths larger, mouths with significantly bigger teeth
* more sloping forward head angles the bone at a lower angle in reference to the sinus cavity and towards the throat and vocal cords.
We often think of the sound of someone's voice coming from their vocal cords, and it does. But the timbre, the quality, the greater sound we hear, like with the guitar, is the result of resonance, is the result of how the sound evolves within its resonance chambers.
The resonance chambers for singing are the nose, nasal cavity, sinus cavity, mouth, and throat. All of those things are phenotypically different than, say, whites. So, of course, the sound is going to be different, distinctively different.
If you change the resonance chamber on a violin or a guitar or cello, the sound produced changes drastically. So the different markedly unique resonance chambers created by black physiology create a unique sound to their voices that is unmistakable, that allows you most usually be able to identify a black singer even without seeing the singer, even when she is singling classically and not in any way that adds cultural embellishments that might cue listeners.