The main goal is usually to exchange favors with other players: If I deal with that problem, will you promise not to attack me next turn? Politics decks use these interactions as their main focus, pitting opponents against each other while they build up for the win
You might take away their fourth land or their preferred three drop but that won't actually do anything for two or three turns which greatly reduces the value of Blackmail being one mana
Not to mention you should obviously be playing it in a deck that revolves around sacrifices and Treasures for peak profit
There is nothing grander than three lands for 3 mana right onto the battlefield
As this presumably takes place after the second Story Spotlight in which renegade leader (and Chandra's long-lost mother) Pia Nalaar surrenders herself into captivity, it can be assumed that Nissa and Chandra have been captured as a result of trying to free Pia
That would make this Black Lotus the second most expensive MTG card ever sold - except that Post Malone recently claimed to have bought a similar Black Lotus for $800,000