The black heart is due to the logs being used green. As they saltwater season in a vertical position, silt stain would be drawn in from the bottom as the sap goes out the top. As for milling, each log would have to be assessed on its own merit. Cracks dont disappear, they just get worse. Turps is always worth a look at, as it stands up to marine use very well. If there is a large parcel of logs, maybe the smart thing to do would be mil a test batch before committing to volume. If you are in a horse area, mill to fence rails as they are highly sought after and quality is not quite so important.
Hope this helps.