Looks to me like the tire is doing just what it suppose to be doing.
To answer your question, I see darky's all the time (I also see dead people, but that is another story).
Seriously, once your pace gets to a place where the tires are really working then you will start to see that. You will see it more with a race tire because they are softer and will leave more rubber on the track. But, the PP has a soft side as well and will leave them.
When you hear people say "the track is green", what they mean is that it doesn't have a lot of rubber laid down on the race line. This is where the best traction comes from, rubber on the track against rubber on the tire.
For example, this past weekend while I was racing I could clearly see thick darky's from the sides of the tires in the T3 carousel at HPT. As the bike leaned over further and further, the darky's would get thinner and thinner as there was less of a contact patch.
So it is not just off the rear tire during drives, it also happens off of the side of the tire. BUT, you have to be working the tire hard enough to cause some "scrubbing" or "sliding" of the tire along the pavement. This "sliding" is also where you get a "feel" for where the level of traction is at. A slow pace will not do this, hence you get no darky's and no feel, only dead people.
Hope that makes sense.