Here's how the event worked:
* You needed to find a special Charizard in a specific location.
* Once you battled and captured the Charizard, it would automatically know Blast Burn.
Important Note: The "Secret Power" event is no longer accessible in these games, as the servers that hosted it have been shut down. This means you cannot obtain a Charizard with Blast Burn in the original Ruby, Sapphire, and Emerald games.
How to Get Blast Burn in Other Games:
* Pokemon Omega Ruby and Alpha Sapphire: You can get a Charizard with Blast Burn via the "Secret Power" event that is still available in these games.
* Pokemon X and Y: You can obtain a Charizard with Blast Burn by trading with a player who has one.
* Pokemon Sun and Moon: You can obtain a Charizard with Blast Burn by using the "Bank" service to transfer a Charizard with the move from an older game.
* Pokemon Let's Go Pikachu and Eevee: You can obtain a Charizard with Blast Burn by transferring it from Pokemon Bank.
* Pokemon Sword and Shield, Brilliant Diamond and Shining Pearl, Legends: Arceus, Scarlet and Violet: You can obtain a Charizard with Blast Burn by trading with a player who has one.
Keep in mind: Trading is the most common way to obtain a Charizard with Blast Burn in modern games.