The answer is simple: defeat the 7th gym leader. That would be Blaine, who is found in the Seafoam Islands. Defeat him to gain access to the 8th.
Blue is the viridian gym leader and red is on top of mt silver once you beat all the kanto gyms
use surf to get to seafoams islands go up the ladder and battle to the gym leader
Brock gives it to you after you beat him in kanto
he is at the sea form islands
face him, click on him, then have courage and believe!
you have to beat jotho then kanto
Blue is the viridian gym leader and red is on top of mt silver once you beat all the kanto gyms
You get the poke flute from a man at the raidio tower in Kanto after you clear the power plant in Kanto and beat the Cerulean City gym leader.
finish the KANTO LEAGUE and beat RED/ASH
use surf to get to seafoams islands go up the ladder and battle to the gym leader
He is in the seafoam islands.
beat the kanto elite 4
blue is the last gym leader in kanto after you beat the elite 4 you can go to kanto by taking the ss aqua in olivine city and go to kanto, battle the gym leaders, and capture legendaries
beat the all the gym leaders
Brock gives it to you after you beat him in kanto
to beat blue you need to defeat all kanto gym leaders then go to cinanwood he will be there
Blue, the last gym leader in kanto region