An Attunement mechanic is added before access to the Bladesong shatters. Similar to how the Tempest's Overloads appear when you attune to their specific element.
However instead of the Elementalist Attunements acting like a weapon swap in function and lengthy cooldown. This one alters the ordering or formation of the Virtuoso Blades. Which gives some visual diversity to how the Blades appear while granting different defensive states designed to counter specific playstyles. Functionally it would be like attuning between different Herald's Facets or Signet skills, where you get a passive effect (attunement) and then an active one (bladesong). The Attunement cooldown wouldn't be like the Elementalist 10 seconds, and would be more like half of a second to avoid skill spamming. I guess it is a kind of weapon swap visually, since they also go from Missiles to something like a Shield in their formations.
Thematically the Attunements work as the Virtuoso shifting through different mindsets or paradigms. Or it could be seen as the Virtuoso tuning themselves to a different sound before activating their Bladesong. The existing Bladesong types of Harmony, Sorrow, Dissonance & Requiem work well because not only do they represent different sounds, they are also linked to a specific emotion or feeling. So the way the Blades exist should reflect these different 'feelings' when summoned, so Harmony has them feeling lightweight and in the air, while Sorrow has them move slower and to the ground, while Dissonance would have them a little chaotic and erratic.
I agree that the literal daggers are a little strange, especially how they don't move with the player. I prefer the organic look of something like the Shiro Revenant healing skill where they are more like shards that float. Maybe turning them into something like mirror shards would work, especially since it would tie the Virtuoso to past Mesmer aesthetics. If they also have irregular faceting then they'd pick up environmental lighting too.