So, let's be clear: you want to show a form with some parts translucent (not totally transparent) but that the "light" that goes through it gets fuzzy, similar to a bathroom glass (to put an example).

But... and here is the most difficult part, you want to it to be totally dynamic. If the figures that are behind move or change, that needs to be updated. If the user is moving your translucent form, the fuzzy background needs to be updated in real time, as the form is moved.

In other words: the same or very similar to Windows Aero.

If I'm understanding that correctly, then, I don't think that such a real time update could be done by "taking pictures" of the windows that are behind.

Do you know of any program that does that? (besides Windows Aero that does it for the title bar)

I would start studying the Desktop Window Manager. Maybe someone with experience with it can tell you, but I didn't see much about it here.
Because that should be handled at low level to be really dynamic. From high level is already difficult to take e single picture without flicker.