you have to go through digglet's cave
you do ecruteack city
Blackthorne city is not in Pokemon Diamond but in Pokemon Heart Gold/Soul Silver
goldenrod city.
it is in the city after pewter
it is in goldrod city
in ecruteak city
By going to saffron city.
goldenrod city in a house there is a lady
its in blackthorn city behind the gym
on the far left in the city
Pewter City, in Kanto.
in viridian city