Oops, missed Midnight's reply.
We've now had words and hammered out that Nurgle Daemon Princes do not have the Mark of Nurgle, and Tzeentch Daemon Princes cannot take Sigils of Corruption.
Goooood, yes. He should be rather less of a thorn in your side, now. Unless your opponent starts fielding him with Biomancy powers, so he'll be regenerating wounds or getting +D3 S/T, etc.