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The Blades of Fire spell doesn't seem to be extendable.
Magic Weapon, Greater Magic Weapon, and Keen Edge (and I'd assume things like Deafening Clang, Bless Weapon, Darkfire, etc) can be extended, and those are spells that do something directly on a weapon.
Is this a bug/oversight, or intentional?
It's a similar consideration as empower/maximize metamagic not being available for cantrips by default (except for a few class/path interactions).
Empowered flare... 
The cantrips Daze, Light, and Resistance work with Extend Spell, though. I just thought it was a little odd for some cantrips to be extendable, but not others! I've been wondering this for a while, lol