I think they are in themselves invisible.
We can see them only because of a lack of other things around them or things getting sucked in, therefore we know they are there and their symptons (i think) can be seen but not them actually.
Therefore my theory is they are not visible.
Just like you all to know I am very impressed with the amount of knowledge around here, we will have to have questions like this more often, move onto debates about religion, and evolution and the origins of life etc. and then when we are ready the meaning of life... haha.
Oh man , can you imagine the pressure inside a black hole ? Don't remember where i read it but it said something like that Earth could be able to fit in one square inch of space due to the tremendous pressure , something like that