Or, since the Black Omen is fueled by Lavos' power, it's simply uneffected by his blasts. It seems to me that the Black Omen serves some kind of usefulness to Lavos. Perhaps, if he's sentient, he craves no more from the Black Omen than the worship it offers him. It serves as a testament to his power. Maybe he intended the Black Omen to travel with his Spawn to other planets and keep them safe? Who the hell knows?