Is there a way to comment out XAML that contains comments?

No, there is no way of having nested comments in XAML.

You could use the mc:Ignorable attribute on your root element, and any attribute or element prefixed with that value will be ignored E.g:

<UserControl ...
   mc:Ignorable="i">

   <!-- Ignore Text attribute -->
   <TextBlock i:Text="Hello" />

   <!-- Ignore entire button -->
   <i:Button>
   </i:Button>

</UserControl>

Note that blend sets the mc:Ignorable attributes value to ‘d’, so you’ll need to use e.g. mc:Ignorable=”d i”