I cannot vouch for the color beyond what the picture shows and the common knowledge that these mussels really do have very blue nacre sometimes. Much as I know (not much) the species can produce both dull and iridescent pearls. Not sure if there is some chemical difference (i.e. nacreous an non) or just structural...
If you might remember, a much smaller, light blue, porcelain-like pearl from the same species, collected on the other side of the Atlantic was posted here a while back. (LINK to thread about blue mussel pearls from Nova Scotia)
LINK to Source at Conchology.be
5.8mm (!) Mytilus Edulis pearl
Did you know such sapphire blue pearls exist at all ? If nacreous, quite a sight... if not, quite the pair for a little conch, I'd say