Question Blend Tree not blending while animation is played?

I've set up a Blend tree that plays a sword swing upon a left click - Attack_Standing has a threshold of 0, Attack_Running is 1.
The parameter is speed, which is set to inputVertical in another script.

It kinda works - when the character is standing and I click to attack, the Attack_Standing plays, when it's walking, Attack_Running plays.

Now when I start walking while Attack_Standing plays, or when I stop while Attack_Running plays... well, the animaiton just finishes, with the character either running on the spot or surfing across the floor like it's slippery.

Lowering the parameter of Attack-Running to something like 0.1 doesn't solve this.

What am I doing wrong?