I'm assuming that you mean Blaine, the Cinnibar Gym Leader. Right on the sign next to the Pokemon Center in Cinnibar, there is a message stating that the gym moved to the Seafoam Islands. The bottom island is where the gym is located, but after entering it, you need to climb the ladder inside the first room.
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he is at the sea form islands
The answer is simple: defeat the 7th gym leader. That would be Blaine, who is found in the Seafoam Islands. Defeat him to gain access to the 8th.
Because Misty is not at her gym, you have to find her up at Route 25.
face him, click on him, then have courage and believe!
Defeat all the Kanto gym leaders. While you are able to do most of them in any order, Blue can only be fought if you have the other 7 Kanto gym badges.