blackened pos terminal

I recently replaced both battery cables with new ones, a few days later i popped the hood and found the pos terminal was black, like something had burnt. my in car voltmeter shows right about 14 volts when cruising and about 14.6 right after start up. I have felt the terminals while the car is running and it does not get hot. Any ideas what could have caused this?

Its in my 75 duster and the only electrical mods under the hood are adding relays for the headlights, but those weren't even hooked up at the time.