After arriving at Mahogany, deal with Team Rocket at the Lake of Rage, Lance will then come and you'll have to fight more Team Rocket Grunts and what-not. After you can challenge the Mahogany Gym Leader; Pryce, if you succeed in winning you'll be able to pass through the right where the man was selling those bars, after go through the Ice cave and you'll be in Blackthorn.
go to blackthorn city and fart
you just go north of blackthorn city and beat the gym leader
you can go to blackthorn city
You have to go east of Mahogany Town then go through the Ice Cave. When you come out of the Ice Cave, go south to Blackthorn City.
go to blackthorn city and press a on the weird looking tree
After you beat the gym leader in Blackthorn city. She will tell you to go there.
I believe you go to the Move Tutor in Blackthorn City.
You have to beat team rocket in goldenrod city before you can go to blackthorn city.
From mahogany city go up to the ice path and cross it.At the end is blackthorn city.
Blackthorn City is located east of Mahogany Town. To get there, head east from Mahogany Town until you reach the Ice Path. Go through the Ice Path until you find an exist, which will be Blackthorn City.
you don't go that way, instead you go though dark cave (you need rock smash) to reach blackthorn city
You must go through ice path at the side of magouthy town after you have beaten the radio tower.