the fact that a person is closeted gay, or that location data revealed exactly when they were with a mistress

Does TikTok have private messages? Any platform with "private" messages can collect blackmail. Tucker Carlson was fired over two texts (or emails, I do not recall).

They may be blackmailed for watching forbidden topics like Russia friendly channels. Or explicit material if TikTok has it.

They may be blackmailed if they are in the wrong social network if TikTok has such a thing.

Literally there are countless things. Firstly, the app is pinging TikTok servers all day so as you move throughout the world, your IP is changing and betraying your location. Your location reveals your social graph and movement patterns, whom you associate with, and where you spend the night.

People watch all sorts of content on TikTok including sexual/sensual content. While they are watching alone in their rooms, all their usage patterns are recorded in intimate details and they reveal all their sexual proclivities. That is quite easy fodder for blackmail.

Health and financial information can often easily be gleaned from people's watch history. If you know people are struggling with their mental or physical health, or are having financial troubles, these are all things that can be used as leverage for blackmail, persuasion, and deceit.