Strange black box appearing in wpf application

Solution 1:

This is a new feature as of VS 2015 Update 2 XAML Diagnostics. This shows up only if you are debugging the application.

If you really need to disable this, there's an option in the Tools > Options > Debugging > General - Enable UI Debugging Tools for XAML > Show runtime tools in application

Solution 2:

This is new (and, actually, useful) feature in VS 2015 Update 2 (In-App Menu). It allows you to pick visuals directly from the screen and switch to visual tree viewer (see "Live Visual Tree" toolbox).

It affects debug mode only.

Solution 3:

As the others have said it is a UI Debugger and can be very helpful. You don't need to go to the options menu to disable this though. There is a button that toggles it on and off in the Live Visual Tree tab.
See image UI Debugger toggle - visual studio