Arrive and look around
Blackthorn waits beyond Ice Path 1F like Johto's last locked door. The Gym, Dragons Den 1F, Move Deleter, Mart, and cliff roads are all built around the idea that you are almost ready, but not quite done.
What to do here
Challenge Clair, then follow the city's tradition into Dragon's Den before the badge chapter truly closes. Check the Move Deleter if your team needs old HM moves cleared, read the signs around the Dragon's Den, and use the city as your rest stop before deciding whether to descend Route 45 or push toward the League.
Return with a reason
Return for Dragon's Den cleanup, the Move Deleter, late Johto shopping, and the mountain loop through Route 45 and Route 46. Blackthorn feels severe, but it is one of the most practical late-game bases in Johto.