Dual blades daggers for all classes?

Blades are a new equip type and are available ONLY for the Dual Blade job to use. There's a new slot in the equip menu (that only shows up to Dual Bladers) for them.

And I really doubt that the "common" ones are usable on anything but DB characters.

Yes. Dual Bladers get a sort of 'new' equip window when they equip blades, exclusive to them.
NO other class can wear the blades, even if they met all the requirements except jobs.