D&D 5E Blur vs Shadowblade for a Bladesinger/Rogue multiclass

[ECMO3]

quickly tire of you spending two full rounds doing no damage so that you can be the guy with Mirror Image, Bladesong, AND Blur all going when you already have Shield and bonus action disengaging to fall back upon.

I would not do that. Mirror image would either be precast before opening the door, or cast in battle after the first turn as a result of losing concentration or getting low on hps or something else.

The only time I would cast it after initiative but before attacking is if I was using Hex for concentration, which is often already up when a battle starts and is a bonus action cast if it is not.