1 Adam: There is some light down here. My eyes are adapting. I can see things now...
2 Jamie: Me too.
2 {Shapes appear out of the gloom. A skeletal figure with a bargepole is standing near them in the boat.}
3 Adam: Gah! Who are you?
3 Charon: Charon, the Ferryman.
4 Jamie: Charon! Adam, do you know what this means?
4 Adam: "Black as Hades." Busted!

Charon hasn't been around since he helped collect Dr No back in #1661.

And so now you know what's underneath the Infinite Featureless Plane of Death.

Or at least, what's under it this time.

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2018-10-25 Rerun commentary: I just did a Google search to find some background on the origin and usage of the idiom "black as Hades"... and didn't find anything. There are plenty of hits for the idiom itself, but no article or lexicon entry discussing the idiom and its origins. This from phrases.org.uk is about the best I could find, and it's not very informative at all, other than to catalogue that the phrase exists and (like all the other "black as ..." phrases) is simply an allusion to Hades conjuring up images of darkness.

If anyone wants to research this phrase and write an article about it, I'll gladly link it here.