Blissey is among the new Pokémon that were recently added to Pokémon GO as part of the latest Johto-themed update for the game. A number of players have noticed that the Pokémon has been taking over Gyms all over. Thanks to its bountiful supply of health points, Blissey can prove to be very challenging to take down. In Pokémon GO, Blissey can be evolved from the rare Chansey with 50 Candy.
Source: Kotaku
Pokémon Blog 