So, I was wondering, the spell shadow blade creates a weapon, right? And kensei Monks can make a weapon a Kensei Weapon, right? So, does that mean Kensei's weapon could be a Shadow Blade? Not belonging in this thread but... Could you do the same with a pact weapon?
There's a couple of reasons why Shadow Blade Pact Weapon wouldn't work or wouldn't be practical.
The ritual to turn a magic weapon into your Pact Weapon takes 1 hour, and Shadow Blade only lasts 1 minute. Even with some multiclassing into Sorcerer for Extended Spell, there's simply no way to make the weapon stick around long enough to complete the ritual.
Even if you could somehow extended the spell's duration by x60, you'd arguably be making that specific shadow blade your Pact Weapon, not all shadow blades you might conjure.
Also, given how dependent Blade Warlocks are on Pact of the Blade-focused invocations, you really don't want to make your Pact Weapon something that you can't have full time anyways. You'd be better off using a different weapon as your Pact Weapon and using Shadow Blade as your second weapon for two-weapon fighting.