I think your example is running pretty good, but may be a tad on the rich side. Nothing much to worry about. If it were running a tad bit leaner, the plug would have more of an tan/ash color to it. When reading the plug, don't pay as much attention to the bit at the top of the threads. This area is prone to have some minor carbon build up no matter how well the plug is running. This is because of it's location and proximity to the head. This area will run a bit cooler as well as like to trap a tiny bit of fuel which causes the carbon deposits. The electrode and ground electrode are the two areas you should concentrate on.
As for other examples and what to look for, here is a chart which shows pretty well a good range of what spark plugs might look like.
I had put this out in another question a while back. While the answer covers this question, the questions really don't line up.