XAML files are XML files, and XML is less forgiving than HTML, so you will need to make a copy of your element with the tag and comment it, and remove the tag from the original element.
There is no commenting/uncommenting button in expression blend, if you want to try to comment your code block, you can type include the below symbols in the begin and the end of the code block manually
<!-- your code comes here.. Next Line-->
See a sample screenshot below
To comment in Visual Studio
Select the lines,