If you have the HM Surf and the TM rock smash you can find your way to get close to blackthorn city however you can't get there by using dark cave only by ice path can you get there.
Beat Team Rocket in the Goldenrod Radio Tower, then you can go to Blackthorn through Ice Cave, which is on Route 44, which is the route that leads out of Mahogany Town.
you have to get through the ice path
ya must get trough icefall cave in any of the 3 games:silver,gold,or sliver
to the right of mohagony town where a bird trainer will give you rematches. Ends at Black thorn city.
It is in the dragon cave behind the gym but you need to beat the gym leader first
Clair is at black thorn city but first you need to get through ice path and route 44
Beat the 8th gym leader which is Clair in Black Thorn gym obviously in Black Thorn city.
you have to have beaten the elite 4 and givenh celio on island one the ruby you go to island 4 and find hm 7 waterfall in waterfall cave
if you come out of the entrance on the top right of your map then just go left across a bridge and then up across another bridge but if you come out at the bottom right entrance on your map then you cant get there
You can get it in Dragons den in black thorn city.
after you kick out team rocket in goldenrod city and make it through ice cave
you beat clay(easy with dewott)go through route 6 and chargestone cave