Darksend is a much bigger feature than what it seems to be in the windows wallet (just a checkbox in 'send' tab currently).
Instead of having this checkbox inside the 'Send' tab, I propose the following:
1) remove 'Use DarkSend' checkbox
2) add a 'DarkSend' tab between the 'Send' and 'Receive' tabs*
3) this new tab looks and functions exactly the same as the send tab but it uses darksend technology when submitting.
This will be much better UX to differentiate this technology from standard technology.
* You can also consider putting darksend tab before the send tab.