dark plug on cylinder 6

Good day. On my 71 Val 4 plugs one through 5 look perfect. Cyl 6 is pretty black. Clean where it's firing. It looks dry.....but I don't see how I could possibly have a rich condition. I even pulled the plug on the top of 6 intake tube and had a look. It's not wet with gas even when sitting figured maybe float was high. But the car starts runs and drives great. Running a points driven t.f.i. module with a high ouput coil. Two sets of plugs have done this. Any ideas? Or is it oil...