Hello MetricZero,
I have been running some tests and am not getting the same results you are reporting. I believe the issue you are experiencing has to do with your set up. Essentially if you want to blend from one post process volume to another with a clean linear fade, you need to have and unbound (global post process), and a bound post process volume.
You would then want to move towards the bound post process volume, and either use the Blend Radius to blend linearly to the bounding box, or use the Blend Weight to blend from 0 to 1 while inside the bounds. Below is an example of the scene I tested with the results of my blending.
Blend Weight 0.0
Blend Weight 0.5
Blend Weight 1.0
Notice how I am inside the volume, it has a higher priority than the Unbound Post Process volume, and when sliding the Blend Weight, I get a gradual and linear change between the Unbound effect and the Bound effect.
Post Process Material
For my material I simply multiply the scene by a color to get the color shifts. I then instance my material and assign a unique instance into the blendable for each volume. Now I am free to use the Blend Radius, Blend Wight, or the Color Overlay scalar parameter to properly blend in the desired effect/color.
Let me know if you have further questions or need additional assistance.
Thank you,