I'm planning to buy Pokemon X/Y, but I want the Black mega evolve of Charizard. In which game does Charizard evolve into the black mega?
The black Charizard (Mega Charizard X) is in Pokemon X
You can get Mega Charizard X in Pokémon Y as well, assuming you have the appropriate megastone, or trade it.
Charizard has 2 mega evolution stones, and the one you get in pokemon X will make Charizard black.