I just started pitting these for jam and about half have a darker area in the interior which has a firm texture with crystal grains the size of chunky sea salt. Some of this is around the pit, some is not. The crystals are crunchy, clear and tasteless. The darker are tastes fine, just a funkier texture. The plums are a tad under ripe - none are over ripe. No sign of disease or insect on the skins.
I've used Italian prune plums for years and have never seen this!
Please solve my mystery!