From the game's encyclopedia
If you know another Character's Criminal Secret, you may use a Blackmail Character Interaction to potentially gain a Hook on them.
The strength of this Hook varies with the severity of the Secret, which in turn is often determined by Doctrines.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Blackmail is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "If you know another Character's Criminal Secret, you may use a Blackmail Character Interaction to potentially gain a Hook on them." It is part of Hook. The game also links it as Blackmailed and Blackmailing. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.