You can get a black background by using higher shutter speeds, ie 1/250
I usually like a brighter background with a dark subject, but being as the light was fading fast I was at or around 1/15 sec for correct exposure, trouble is at that speed you'll get camera shake or the bug will move, my actual shutter speed for the shot was 1/80 after I bumped it up a couple.
not taking into consideration the flash, with flash set on ETTL it applies the correct amount of power to expose the shot.
The other way is to make sure you have light fall off as is the case here, there is nothing but trees in the background a fair distance away and there is no way the flash would illuminate those trees in the background.
Have to be aware of your background composition.
Hope this helps.