Are you sure that it's meant to be black conjunctiva and not black sclera? Conjunctiva is really thin ,vessely and covering the visible eye like a blanket, instead of encompassing it, so it being black would be really spotty at best (if at all, as it's so thin and transparent for a purpose) and if it actually were opaque black, the vampire would be blind.
Every other mammal (or even animal) I know of with non-white sclera, obviously has the colour in the said sclera.