You wake up Snorlax and go through Diglett's Cave.
you do ecruteack city
After that guy has upgraded your pokegear, go to vermiion then go right, you will see a sleeping snorlax. Go to the radio on your pokegear, drag the circle to the top star in front of the snorlax, you will battle him, he will move. At the end, go through digglets cave and go out, and you will soon find peweter city. Go down from peweter and you will soon find veridian forest.
Blackthorne city is not in Pokemon Diamond but in Pokemon Heart Gold/Soul Silver
it is in the city after pewter
goldenrod city.
it is in goldrod city
in ecruteak city
By going to saffron city.
goldenrod city in a house there is a lady
on the far left in the city
its in blackthorn city behind the gym
in viridian city