How do you get blue flute in Pokemon fire red?

You must trade it from Pokemon Ruby/Sapphire/Emerald, where you must trade soot to get it.

in cinabbar islands

I forgot exactly who gives you the Pokeflute, but I know it is definatly someone in Lavender Town. Mr Fuji, I think. From pokemon tower top floor

you get the pokeflute when you rescue mr. fuju on the top floor of that big building with all the mist and ghosts, i don't remember quite what its called but you can use it to catch snorlax...

I'm pretty sure one of oaks little aides gives it to you if you've caught a certain amount of Pokemon.go find oaks aides they are usually upstairs in those passage rooms