My understanding is, as light can’t escape the black hole, it doesn’t need to be vantablack as you can’t see it anyways 
We’ve only been able to “see” the black holes due to one of like 3 things:
the math of the orbits of some of objects we suspect are around one
we can see the actual accretion discs around them and/or escaping xrays from just before the event horizon
we can see possible light/spacial diffraction when something passes in front of them
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