Aye, I think perhaps it's the ambience setting of yuor planet's materials? Perhaps post a screenshot of the Material/s you're using? To be dead black, the ambience LEVEL should be zero, and not running through any channels? Just some ideas... Also, the way we used to do it was to use a negative light on the night side. Set it to ranged, have it's power equal that of the positive light (if you're not using the sun, anyway)... Just a workaround!