after you kick out team rocket in goldenrod city and make it through ice cave
Aliza town
you can trade pokemon from heartgold to black but you can only transfer the pokemon in heartgold not trade it from black
in the radio tower
Goldenrod City
in cerulean city in the gym idiots
You must be referring to BLACK City, as Blackthorn City is exclusive to the Johto region (only found in Pokemon Gold, Silver, Crystal, and Pokemon Heart Gold and Soul Silver). The amount of people in Black City changes each time you link-up on WiFi.
Aliza town
you can trade pokemon from heartgold to black but you can only transfer the pokemon in heartgold not trade it from black
Blackthorne city is not in Pokemon Diamond but in Pokemon Heart Gold/Soul Silver
You cant.
it should be in olivine city
blackthorn city
In fuchsia city.
by being awesome
dodo
You have to get it from Pokemon soul sliver or heart gold or trade.
If you Poke Transfer from Heart Gold to Black, the Pokemon should be in your box