You now have to go back to Goldenrod City and take back control of the Radio Tower. Just head to the Radio Tower, and clear the left wing (Pokemon around lv.30-35 is recommended for this portion of the game, btw).
head to the Underground Passage (Where you can dress up your Pokemon and get the Coin Case), then trek through a puzzle down there. You'll wind up in the basement of the Goldenrod Department Store.
Once you save the Radio Tower's director, he'll provide a key that gives you access to the right wing of the tower! Clear the right wing, and you're ready to head toward Blackthorn.
The freedom and fate of all Pokemon rests in your hands, Young Trainer. Onwards! =D