Odd spark plug wear

[Barrnone]

I am hoping some of you have seen this kind of pattern before and can explain to me where I should be looking for a cause. This is a 350 TBI motor that has gradually started to get intermittent misses, getting worse over time. I pulled the plugs and all 8 of them have a dark gray spot going down the insulator, some with deposits on the spot also. I have attached a couple photos to show what I am trying to describe. These plugs have almost exactly 10,000 miles on them. Thanks for any help.