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You will have to use the talk page to explain this, because there is no direct way to answer it without assumptions. The term "Black Vocaloid" is too ambiguous.
'The someone' could be referring to Hatsune Miku being portrayed with black hair and black clothing, or they could be mistaking her for Zatsune Miku or Black Rock Shooter.
- Zatsune Miku is a derivative based on Hatsune Miku. Her hair is black and her eyes are red.
- Black Rock Shooter is not a derivative of Hatsune Miku. Her hair is black and her eyes are blue.
- Black Miku is a term by Japanese fans who use it as a general term for Miku when she is portrayed with alternate colors.