What are the special Blades Gramps mentions (and how are they unlocked)?
Solution 1:
It would appear that special Blades are rare, "named" Blades that can be gotten from any core crystal. Some specific crystals give these Blades, but they can also be gotten from common and rare core crystals - rare ones have a higher base chance to give a "special" Blade.
Special Blades have their own unique intro and graphic. Some unlock specific merc missions (Azami comes to mind) where they are at the forefront.
A listing of every unique Blade (story and special) can be found here - spoilers beware.