I am very new to Expression Blend but can't find any solution with google. I would like to style a Menu and it's items. But how can I see the actual look of the PART_Popup? The IsOpen Property is bound to something like {Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}. It seems I am only looking at something complete invisible :). There is also a SystemDropShadowChrome but I actually can't see this thing. The eyes next to all the items are active. And even if I change some background colors all that I see is just a black transparent rectangle.
Is this a toolkit control (dropdownmenu) or a telerik (radmenu) or something? If you've already broken out the template (eg; Right click->Edit Template->Edit Current/Copy) Then there's a couple things you want to try to find it since sometimes they can be oddly embedded.
Go to the root level of the control, try Right Click->Edit Additional Templates->...
If that' doesn't provide your part_ look in your broken out template resource you've created and inspect your object tree. You're looking for embedded controls appearing as objects. Right click the embedded control in your object tree and edit its template (Current or Copy)
You'll potentially sometimes have to drill down multiple layers within a template to expose the objects you wish you to edit. If you specify which control you're working with I'm sure we can tell you exactly how to get to where you need more efficiently. Cheers!