The WIN says that The Mound (the 2x2 terrain with 1 square of blocking and 3 squares of elevated) is both Blocking AND Elevated.
(https://win.wizkids.com/bb/viewtopic...8ff2d4786cad3f)

Now that ruling is certainly weird, as that means a figure can be on the Elevated but that means they're also occupying Blocking.

Here's my question (I've asked there, but wondering what you think) - would Carnage Silver Surfer, who has IT Elevated, be able to shoot through The Mound? All of it? Only the Elevated squares?

It is effectively treated as blocking and elevated for effects looking for both. The way that ruling is written, the entire 2x2 is effectively elevated blocking that you can stand in.