ブラックホール (burakkuhooru) is phonetically based off of the English term and has the same meaning.
穴 ana
You can't see the black hole but you can see its inflence on its environment. (You can see matter that is sucked into the black hole)
its kuro
Nothing, unless a black hole comes very near to us. By the way, you shouldn't say "the" black hole, unless you make it clear which black hole you mean. There are many black holes.
A black hole has infinite density.
kokuchou
kuroji
kuroari
Kurotsuki
Kokushibyou
kuroikokoro
Kuroi sora is black sky.