It's east of Mahogany.
im not too positive about this answer but im pretty sure you cant get to blackthorn city through the dark cave
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 through the ice cave
blackthorn city outside of ice cave
in the back of blackthorn city their is a cave where they are found
you just go south of blackthorn city
You get it in the Ice cave before Blackthorn City
in the dark cave near blackthorn city
go to blackthorn city through ice cave
You find blackthorn city and beat the gym leader
You have to go past the ice cave in route 44 and then you will be there.
By beating the gym leader Clair in Blackthorn City.