Blaine's not gone, the key to the mansion is missing. You have to find it. The key is in the Pokemon Mansion.
The Pokemon Mansion is an old mansion just above the Pokemon Center. Once you're inside, head straight and then up the stairs, which will take you to the second floor. You'll find Zinc and Calcium on this floor. After picking up the items, head to the northwest corner of the floor, and take the stairs to the third floor. Go all the way right and pick up the Iron. Go back and press the switch on the statue. Go down, and you'll see a scientist. Go past him and fall through the larger broken floor. You'll then fall into another section of the first floor. Fight the Scientist and go near the stairs. Pick up the Carbos, and go downstairs into the basement.
In the basement, go into the room to your left, and switch the Mewtwo statue so that's its eyes aren't glowing. Head up into another room to pick up TM14 - Blizzard. Flip on the Mewtwo statue in the other room to pick up two items. One of them is the Gym Key, which unlocks the Cinnabar Gym; the other is TM22 - Solar Beam. Now that you've gotten what you came for, it's time to leave this mansion and go to the gym. -pokedream