i'd imagine it's something like blood coagulation (sp?) under the exoskeleton..maybe had some form of internal damage, that would be my theory anyway since the exoskeleton looks intact.
could just be like when people see their ghost shrimp turn cloudy before they die. my cherries turn bright red or orange before they die. some even turn dark brown with red tint. so its all normal i guess when they are at the point of no return.