Keep working on your jetting until the gray line is about a pencil width thick. As Jaybird is eluding to, that plug looks too dark for a proper WOT chop. Warm the motor up with a used plug and change to a brand new one for the check. Remember to run 30 seconds WOT, 4th or 5th gear on a slight uphill and cut the throttle, pull the clutch and hit the kill button all at the same time. Even a slight delay on any of these will cause the plug to read incorrect. If you don't have a hill to put a load on the engine, try riding the rear brake pedel for resistance. WOT means WOT and backing off won't give a correct read. I've not heard of a stock two stroke that could over rev to the point of exploding and I wouldn't worry about it. If you just can't bear to do it to your own bike, have a friend do it for you. Most friends are more than happy for the opportunity to damage someone else's bike. (That last part is a joke,, your bike can handle a WOT for 30 seconds)