Grey area around spark plugs!!!

[Dan]

Has anyone ever seen grey colored areas on the cylinders around the base of spark plugs??
The airplane is a Piper PA-28-161 Cherokee Warrior with the lycoming engine. I dont know if these even came with any continental engines?
The area around the base of the plugs has a grey color to it. This coloration has been on the cylinders for some time. The last 100Hr was even signed off as airworthy, but this is obvoiusly not normal. I suspect cracks or a problem with the threads. The 100Hr is due again and in my view should not be signed off. Advise is very welcome(and needed)!!!