I agree that's the only way to know for sure.I agree, the only way to know if it's an allelic mutation(of Oocyan) it's with a full Quantitative Trait Locus (QTL) Analysis.
But since we do know that the usual Oocyan mutation is linked to the pea comb locus, a few generations of test matings with pea comb birds could give some indications. If the blue egg gene she has is linked to her not-pea comb, it's more likely to be either the familiar mutation or another allele at that locus. If the blue egg and the not-pea comb assort independently, then she's most likely got something new.