I sometimes commute to and from work, its only 4km each way, the bike does fine. The plug is not you're solution.
Stock jets are 130/120 on a less flowing carb, you could still be WAY of. With my ninja ex250 carb I have 110 mainjets, and they are sliiightly rich (tested with lambda probe-thingy), even with an oversixe header, laser exhaust and opened up airbox.
Stock is 130/120, i had 140/130, it was way rich at WOT (15-20% too rich from optimal burn), and I couldn't even feel it, and my plugs seemed spot on, they were never black. I had to have it dynotested to see the real picture.
I even had to lean out my stock carb with smaller mainjets, and lower the needle in the secondary carb for better riding.
Rejetting like this is really fumbling around in blindness, and I will never rejet without beeing able to test it.
The pic you have there might as well be from a bike burning a bit of oil, theres 1000 scenarios that could cause a plug to look like that.
Lean out the mainjets some more, and have the bike tested. Think about how much time you have spent on building and adjusting the bike. Toss in the last few bucks so you can ride and be done with it.
Best wishes