Here's why:
* The Rage Candy Bar is a cheat item: It's a fan-made item, not a legitimate item in the games. It's meant to be a joke item and doesn't actually have any effect on the game.
* Blackthorn City is accessible through normal gameplay: You can enter Blackthorn City by following the main story progression. You will need to defeat the Elite Four and become the Champion of Kanto to gain access to the city.
Therefore, there's no way to get into Blackthorn City by buying a Rage Candy Bar. You need to follow the main story and beat the Elite Four.