Possible to drive Alpha in animation Blend nodes via animation progress?

Is there some way how to drive the Alpha value of animation Blend nodes by progress of animation itself or by it’s curve?
What I mean is something like this (it’s custom blend):


I would want to blend from the “Trans_Idle…” to Destination State Pose in the last few frames when “Trans_Idle…” animation is close to it’s end. I was hoping for something simpler, like to drive the alpha with a curve that I have created on the “Trans_Idle…” animation itself (I mean the curve like the one that can drive morph targets or material).