If by "Black Vocaloid" you mean a voice bank that has a personified avatar who is dark-skinned- then we as a fandom do not have the answer. It is up to Yamaha and the companies who hire designers to make that choice.
Skin tone has no effects on the voicebank itself. Even though there are certain accents that are normally stereotyped to darker skin tones, there is also no way in certain that a singers method of speaking will even transfer into a Vocaloid's voicebank. A singer with a thick accent such as the stereotypical "African-American" or "British Black" accents, may not even show up in the Vocaloids pronunciations, or sound very different to the actual singer themselves.
Take note that the native Japanese people have variable tones of skin colour themselves. Yet, as per the manga style of cartoon design, most of the current avatars are light coloured tones of skin.
Some fan art personify Leon and Lola as 'black' due to their promotion as soul singers and Lola's vocals. And although the genre is not specific, it is known to primarily consist of those who are Afro decent.