Alright, I double checked, and I had saved and quit on the "this is worse than being a medium" text, which is why I didn't unlock the black soul entry. My bad. The talents entry exists and is added to the book at the beginning of day 4, but the link to talents is still in the Mediums page, thus my initial confusion. Sorry for the misunderstanding lol.
(One small thing, the text for the black soul is, well, black, which doesn't show until you hover over it. Except on android, you can't easily hover like that. Would changing the base color of the text to the dark gray that is used when hovering over it instead be possible/desired? Then changing the hover color to a slightly lighter color?)