This appears to be a regression introduced sometime after 0.1.238 (Appears in 0.1.241 onwards).

Currently, the "Effect" Property can not be customised within a BackgroundEffect Behaviour.

See this post:
https://www.noesisengine.com/forums/viewtopic.php?t=3629#p19086

Repro Steps:

1) Create a Studio Project containing a MainPage.xaml, and containing one bitmap image in the Project's directory.

2) Insert the bitmap image into the Root Grid, and give the Image Element a name of your choice..

3) Introduce any shape above the Bitmap image added in Step 2), and give this Element a name of your choice.

Creating a Behavior
2) Navigate to the Interactivity Panel [Ctrl+3], and jump to the 'Behaviors' Tab.

3) Pick the Shape Element to be used to blur the background Element.

4) With the Shape Element still selected in the Navigator, press the '+' (Plus) icon at the top-right of the Interactivity/Behavior panel to add a new Behavior to the rectangle.

5) Select 'BackgroundEffectBehavior' from the ContextMenu that's popped out.
(This will add the rectangle to the Behaviors panel, and will populate the Properties for the Behavior on the right of the application window.)

Configuring the Behavior
6) Hover over the 'Source' field in the Behavior's Properties, and click on the little 3-Dot menu icon that appears, then click "Create Binding".

7) Select which Element we would like to target to be blurred. To achieve this, whilst remaining within the Binding section of the Behavior's Properties:
8) Select the "Name" tab, and then in the 'Element Name' dropdown list, find what you named your background Element in Step 1).
9) In the 'Path' field where by default it says "Not Set", simply delete that text from the field

Applying the Blur
10) Still remaining within the Behavior's Properties, pick the BlurEffect from the 'Effect' section.

11) Attempt to Expand the "Effect' Dropdown.

Observe how the section cannot be expanded.

Expected Results: The Effect's Properties can be expanded, to then be customised.