It sounds as if the constant power feed enters the light box first There is another cable from the light box to the switch. They probably connected the blacks together in the light box, connected both black and white to the switch, (should have taped the white with black tape on both ends). Then connected the white taped black to the black wire on the light fixture. Hope this helps. Basically all they did was carry the power to the switch with one piece of sheathed electrical cable cable - Just curious, did they tape the white wire black?