First, you have to have already done the following:
Once you have done all that, the man to the left-most point in Mahogany Town should let you through. If he stops you to buy a RageCandyBar, just buy it. You'll need it later on as the game progresses.
Continue heading left and (after most likely facing a series of Trainers) you should have reached a cave called Ice Path (make sure you have a Pokemon with the move Strength with you and [optional] some Fire types to handle all the Ice type Pokemon in Ice Path). You might want to catch some of the Ice type Pokemon in Ice Path because the gym leader of Blackthorn uses Dragon types, which are weak to Ice.
Once you make your way through Ice Path, you should be in Blackthorn City.
Hope this helps!
its in blackthorn city behind the gym
Clair, Blackthorn City
Its right behind blackthorn city
Yes.
blackthorn city
Go to mahogany town then to route 44 into the ice to Blackthorn City. It worked for me.
you have to beat team rocket in goldenrod city
The move deleter in Pokemon silver is located in Blackthorn City.
To Blackthorn City.
you just go south of blackthorn city
in the dark cave near blackthorn city
walk through the door