You need to acquire the Secret Key that can be found in the basement of the Pokémon Mansion and then you can use the Secret Key to enter into Blaine's Gym.
bottom floor of Pokemon Mansion
its actually the 8th gym so you need 7 badges and then it will unlock
Find the secret key in the Pokemon Mansion,( Next to the gym)
The first gym is in pewter city.
The 5th gym is the Fuchia city gym Gym leader Koga
up the stairs in seafoam island
bottom floor of Pokemon Mansion
Head over to Seafoam islands where Articuno is located.
With the secret key which is found in the Pokemon mansion.
Blaine's Gym is in Mahogany Town.
Go to seafoam islands, and climb the latter.
You have to beat the 2nd gym.
revive a pokemon from a fossil and beat the fire gym leader Actually, you need to get the key from the Pokemon Mansion. It wouldn't make sense to require you to beat the gym in order to unlock the gym!
its actually the 8th gym so you need 7 badges and then it will unlock
awww by beating the 7th gym
Find the secret key in the Pokemon Mansion,( Next to the gym)
you go to the Pokemon mansion next to it and at the very end of it is the key to the gym