I would guess that the orginal plaster wall had a bonding agent applied ( Plaster Weld )the dark blue lines maybe score lines in the orginal plaster. Over the bonding agent a skim coat was applied.The crumbling plaster could be the results of the bonding agent having been applied.Is this wall the interior side of an exterior wall if so the bonding agent may have caused a moisture problem with in the base coat plaster.