These are baroque to semi-baroque akoya pearls. The color appears to be natural, but it's difficult to tell with certainty from the photos.
If the color is natural, it's caused by organic material deposited on the nuclei during the recuperation stage after the grafting procedure. The color of the organic matter is actually a dark brown, which shows through the nacre as blue. If the color is natural, the nacre appears to be substantially thick as I don't see any tan or beige showing through (indicative of thin nacre on blues).