According to Hodgdon's site 5.3 gained of CFE is the starting load for 115 grain bullets. I suspect that the blackening is caused by the lighter load not giving a solid seal of the case wall against the chamber. I have the same issue with the .45 acp loads that I run for IDPA. In my 9mm I run near max loads and nickle cases. I don't see any blackening of them. Your results should be similar.