Pokemon Heart Gold and Soul Silver - Walkthrough - Blackthorn City Pokemon Heart Gold and Soul Silver: Walkthrough - Blackthorn City
Blackthorn City is pretty small. Heal up at the Pokemon Center. The MOVE DELETER is in this city, which may be useful if there is an HM move you've been wanting to get rid of.

There is a Pokemon trade, a DRAGONAIR (female) for a DODRIO, in the top right house.

After you visit everything in the city, go to the GYM, which is in the top left part, with a body of water behind it.
This walkthrough was written by MKaykitkats, used with permission.