I'm with Ron on this one. The top two on the left look to have a little blow-by on them. They were probably a bit loose and the seal wasn't quite perfect. The other four look very good and were similar to what all of mine looked like at 110k miles when I changed them. The best thing you can do is have an accurate torque wrench and ensure that the plugs are tightened to spec at 13lb/ft. It's something you should check once a year or so. I did mine at 14lb/ft because I don't like odd numbers.
I never used to be that way until I started dating my wife many moons ago. She is the same way as you, fastdrvr, in that she can do even numbers or multiples of 5. 1, 3, 7 and 9 are no bueno. She'd yell at me for having the radio volume at 9 or 11 or having the A/C on 69 or 71. After 11 (can't wait til next year for it to be 12) years, I now find myself being preconditioned to shy away from odd numbers. My auto climate control is almost always on 70, the radio volume is on 8 for normal listening and 12 or 14 for louder listening. When we were buying a home, we shied away from a potential candidate simply because the house number was 217. The one we bought is 102. When we picked a home phone number, our choices for the last four digits were 2071 or 6080. Guess which one we went with...
Ugh. I guess it's a good thing there are six cylinders, four wheels and two headlights on both of our vehicles!
I'd say its just a little bit of blow by on the inside seal on the plug hole (the lower rubber gasket in the head cover, not the one around the top) and it's dark like that from the heat transferred thru the plug to the portion of the insulator that wasn't covered by the coil boot. In other words, the plug itself was tight enough, the seal just allowed some oil to get thru... by some, I mean just a little bit, and it cooked off. I noticed I had a really small amount showing on my new spark plugs that were changed 3k miles ago, when I was removing the head covers to adjust the valves.
I'd bet the plugs were all still fairly tight when you removed them?
They look fine though, no chipped or burnt/melted electrodes and the color of the plugs are all similar, nothing jumping out as bad to me at all. I will say that looking at 100k plugs on a modern engine vs. 15k or 30k in a vehicle with a carb or early fuel injection is a whole lot different, and usually a whole lot more telling as to whether the vehicle is running lean\rich and with sufficient spark.