You raise a good point. I'm assuming that Jack is the Face of Boe and he does die in that episode. And there is a problem with that isn't there? Because its not at the end of eternity. The earth burns to a crisp when the sun becomes a red dwarf around 7.9 billion years AD(in Who verse). So maybe the darkness he sees means he isn't done yet. Maybe his soul goes into some kind of stasis or limbo or purgatory in time and space until the universe ends.