There are many ways SA500 is different. While SA500 is purpose built for NAS, WD blue is a consumer grade SSD.
Endurance on WD SA500 is much higher than WD Blue. On a 4TB drive the endurance is 4 times that of WD Blue ( Blue = 600TBW, SA500 = 2500TBW). A Better endurance means a much longer lifecycle of SSD.
MTTF of WD SA500 is much higher than WD blue.
WD SA500 is purpose built for NAS. WD blue is not.