Whether the target creature is legendary is checked only as Blackblade Reforged's first equip ability is activated and as that ability resolves. If the creature somehow becomes nonlegendary later, Blackblade Reforged remains attached to it.

"Equip [quality] creature" is a variant of the equip keyword. "Equip [quality] creature [cost]" means "[Cost]: Attach this Equipment to target [quality] creature you control. Activate only as a sorcery."