I've never personally seen it happen, any time my group gets dark, we usually get screwed because we don't usually have ei or SoD so we resort to spamming energy drain and sunburst on the hope that it would trigger and while energy drain has it's equal share of procs, we have not seen a sunburst darkness proc in the process of upgrading 2 emp weapons.
If it is true, it is exceptionally rare and I won't believe it till I see some proof that couldn't just be someone mistaking a grellow discernment for a red or if I actually manage to do it myself.
Also, on the comment that it may have to actually deal darkness damage to proc, then the same would be true of the light side of things as well but we've never had any trouble procing a light stagger with sunburst.
I dunno, but I'd like to find out for certain so I can stop wasting my time.