An Ace Trainer will prevent access to the Gym itself until the player has at least seven of the eight Johto Badges and has defeated Team Rocket at the Goldenrod Radio Tower, though there is no legitimate way to access Blackthorn with less than seven Badges.
Blackthorn City Pokémon Gym Leader: Clair
The Blessed User
of Dragon Pokémon
In Pokémon Gold, Silver, and Crystal, the Gym Leader Clair is on a platform in the middle of red-hot lava, and the player must go to the second floor, where a Strength puzzle must be completed. Once the puzzle is completed, the boulders pushed down the correct holes form bridges across the lava, allowing the player to reach Clair. Even after being defeated, Clair does not consider the player ready to challenge the Pokémon League and refuses to hand over the Rising Badge, forcing the player to go to the Dragon's Den and complete a trial before the Badge can be acquired.
Blackthorn City Pokémon Gym Leader: Clair
The Blessed User of Dragon Pokémon
In Pokémon HeartGold and SoulSilver, immediately after entering the Gym, the Gym guide tells the player to return their walking Pokémon back to its Poké Ball. After this, the player has to navigate on platforms floating on lava, using arrows on them to move them and the blue buttons on them to spin them. After three platforms and several Trainers, the player will reach Clair and battle her. Like in Generation II, after being defeated, Clair still deems the player unready for the Pokémon League and refuses to hand over the Badge before the player has completed the challenge at the Dragon's Den.
The Blackthorn Gym first appeared at the end of Beauty is Skin Deep. In Fangs for Nothin', Ash challenged Clair to a battle for his final Johto League Badge. Although he battled hard, Ash wasn't doing very well and was constantly outmaneuvered by Clair. However, before the battle could end, an alarm went off, alerting Clair that somebody had stolen the Dragon Fang kept at the Dragon Holy Land. She told Ash that the battle would have to continue some other time, causing the match to be interrupted.
In the anime, the Blackthorn Gym has a retractable roof, which is used when both Clair and the opponent's Pokémon can use flight in one way or another. In the original series, the Gym battlefield also had a pool of water at its center, but in Pokémon the Series: Black & White, it is no longer there.
Kingdra was used in both battles with Ash. In the first battle, Ash first decided to use Pikachu, but as Pikachu's attacks didn't have that much effect on Kingdra, Ash switched to Noctowl, who struggled against it and was close to being defeated when the match was interrupted due to Team Rocket stealing the Dragon Fang.
During the rematch, Kingdra put up a good fight against Ash's Snorlax, but was eventually beaten in endurance and taken down with an Ice Punch.
Gyarados is Clair's hidden powerhouse. It was seen guarding her Dratini. Later, during Ash's rematch with Clair, it managed to defeat Ash's Snorlax and put up a good fight against his Pikachu, but eventually lost to Pikachu's Thunderbolt.
Dragonair is Clair's main Pokémon. It first appeared as a Dratini, when it was shedding its skin and Clair was protecting it at that time. When Team Rocket tried to steal Dratini, Clair protected it which led it to evolve into a Dragonair. With its new powers, Dragonair easily defeated Team Rocket.
The Blackthorn Gym appeared in The Ultimate Battle…?!. Green entered the Gym to impress Clair, but he and his Charizard were badly beaten by her. As Red and his Pokémon went inside, they had to get past a river full of lava in order to get to Clair. Even though the group was burned in the process, Clair cooled them off with a water bucket and challenged Red to a battle using Dragonair. Despite Dragonair being a tough opponent, Red was able to defeat it with Clefairy and Tyrogue, although their Double Team clones caused some trouble in the end.
In the games, the Cianwood Gym, the Blackthorn Gym, and the Celadon Gym are the only Gyms that require the use of an HM, as part of a Gym puzzle, in order to progress. Cianwood and Blackthorn's Gyms require Strength in Generation II, while Celadon's requires Cut in all generations except Generation II.
This is the only Gym in any Pokémon game where the player doesn't obtain the Gym's Badge after defeating the Gym Leader.
In Pokémon Crystal, if the player whites out due to poison damage while still inside the Dragon Shrine in Dragon's Den after receiving the Rising Badge, they can return to the Blackthorn Gym to receive the reward TM from Clair. However, if the player then returns to Dragon's Den and stands on the tile directly in front of the Dragon Shrine's door, Clair will appear again and give the player another TM24.