Like Photo Stiching but Not Exactly

[Ofnuts]

The Blend assumes that the bottom layer is a background and generates the blend between the other layers, so if your stack is:

Layer3
Layer2
Layer1
Background

Using Blend with 3 intermediate layers will generate (remember that the animation is played from bottom up):

Background+Layer3
Background+Layer2.75
Background+Layer2.5
Background+Layer2.25
Background+Layer2
Background+Layer1.75
Background+Layer1.50
Background+Layer1.25
Background+Layer1

So, a cop-out is to add an additional layer at the bottom as a dummy background, it can be anything because it will be hidden by the other 3.