Blade is the common name given to the series of Swords including Iron Blade, Steel Blade and Silver Blade. "Claymore" and "Greatsword" used to be common names for this weapon series, but these began to fall out of favour after the English release of Fire Emblem: Rekka no Ken and are no longer in widespread use.
Blades were introduced in Fire Emblem: Genealogy of the Holy War as an alternate to the regular Iron, Steel and Silver Sword series with increased might but also increased weight and decreased accuracy. However, it became apparent that the sword users in that game tended to have more than enough Skill and Speed to render the drawbacks of the Blades effectively meaningless, and so in Fire Emblem: Thracia 776 only the Iron Blade was included.
Steel and Silver Blade returned in Fire Emblem: Binding Blade and have remained until Fire Emblem Awakening. The developers attempted to balance the weapons by increasing their weight further and by making them harder to acquire; whether this has worked or not is debatable.
In Fire Emblem: Radiant Dawn equivalent series of weapons were added to lances (Greatlances) and axes (Poleaxes).