Pokemon Blue where is Cinnabar Mansion?

Cinnabar Mansion is located on Cinnabar Island, the largest island in the Sevii Islands. It's not actually in the original Pokemon Blue game. It was introduced in Pokemon Yellow, Gold, and Silver.

If you're playing Pokemon Blue, you can reach the Sevii Islands by using the Sea Route. However, you won't be able to find Cinnabar Mansion there as it doesn't exist in the game.