That doesn't bar him from having been Lawful Neutral at the start of SoD- LE deities allow LN clerics of them. Though there is admittedly no evidence either way.

And LN clerics of LE deities still "ping a paladin's Evil Radar".

If the Dark one is NE with the special trait of being able to grant the Law domain, as opposed to LE, this might be trickier.