IRL their intestines and other major organs aren't fully formed therefore they perish. Typically they end up getting put down soon after born to end any suffering
Blue Diamond Occurs when you cross 2 frames or 2 white horses. It has a 25% chance of happening. Basically a homo white or frame (OO or WW) is what causes this
you bred two whites or two frames and it carried a double copy which makes it an OLW foal, these foals in real life suffer from multiple problems and die within days and are imediately put down