Blissey locations — where to catch

In Paldea (Scarlet & Violet)

• East Province (Area Three) — Walk
• Glaseado Mountain — Walk
• North Province (Area One) — Walk
• North Province (Area Three) — Walk
• North Province (Area Two) — Walk

In Blueberry Academy (Scarlet & Violet – The Indigo Disk)

• Coastal Biome — Walk
• Savanna Biome — Walk
• Torchlit Labyrinth — Walk

In Hisui (Legends: Arceus)

• Avalanche Slopes — Walk
• Obsidian Falls — Walk

Other games (2)

• Sword / Shield — The Isle of Armor (Gen 8): Brawlers' Cave, Challenge Beach, Challenge Road, Courageous Cavern, Fields of Honor, Forest of Focus, Honeycalm Island, Insular Sea, Loop Lagoon, Potbottom Desert, Soothing Wetlands, Stepping-Stone Sea, Training Lowlands, Warm-Up Tunnel, Workout Sea
• Ultra Sun / Ultra Moon (Gen 7): Poni Plains

Blissey — full Pokédex entry