We do, because in terms of writing english black boxes are opaque by their very nature. OP could remove "black boxes" to make the sentence correct. You can't a

We do, because in terms of writing english black boxes are opaque by their very nature. OP could remove "black boxes" to make the sentence correct. You can't actually see inside a black box. That's why it's a "black box". Sure you can probe it, but that's not the same as being "nebulous", a thing you can actually see through sometimes. I expect better of HN.