As covered by Red Dwarf...
"Well, the thing about a Black Hole, its main distinguishing feature, is it's black! And the thing about space, the colour of space, yer basic space colour, is its Black! So how are you supposed to see them. ... We've been in space for three million years and there hasn't been one! Then, all of a sudden five of them turn up at once!"

And the cause of all these black holes?
"Five specs of grit on the scanner scope....the thing is about Grit... is it's black.."