How to wire black and white wires from new light to existing wiring
My working theory is that the old light was built without neutral wires, because the builder gained the kindergarten knowledge that equipment safety ground is typically bonded to neutral back in the main panel.  Therefore (neutral) current can be returned on safety ground if you totally disregard the purpose of safety ground and neglect the very serious safety implications of misusing it. 

Often in a custom lamp, the lamp is 99% creative inspiration by a master of metalwork or other non-electrical craft, and 1% electrical wiring tossed in as an afterthought with no skill in the domain.