Exactly. Your eyes are being fooled. The blur is the same. That it doesn't look that way is an illusion. But...it's a good illusion.
I think that we need to define "background blur" now, because it does not look like the same COC at 200mm and 400mm.
And what about my extreme examples in the previous post? Do you still call it: "my eyes are fooled"?
Background blur is already well defined, you are not using it in its common context. As I said, the towers are both blurred exactly the same, below is an image where I have made the tower the same size from the 200 and 400 shots, they are exactly the same.
You are not referring to how blurred an object is, you are referring to how big objects at different distances are in relation to each other. That is perspective, not depth of field. For a same sized subject and aperture a longer lens shot from further away will give you a bigger apparent background object, hence the illusion of it being more blurred.
You did the operation which affects DOF - you changed magnification - look at: http://en.wikipedia.org/wiki/Depth_of_field