When disconnecting battery cables you should always loosen up and remove the negative battery cable first.
If you try to disconnect the positive battery cable first you risk making contact with the metal of the car and causing a short of the battery which could result in an explosion or cause an arc flash.
If you remove the negative battery cable first you won't cause a short with the battery.
So when removing the battery cables on a car or other vehicle always remove and disconnect the negative "black" battery cable first and place it out of the way of the battery.