We may have a misunderstanding of terminology. Generally a battery is labelled as "+9V" and "GND" (or 0V), it doesn't have a "9V-" terminal (though you could call the red wire 0V and the black wire -9V but that's not relevant here).
+9V is the red wire, GND is the black wire. Connect +9V (red wire) to a diode anode, diode cathode to Vin, then Arduino GND directly to battery GND.
If you have something else in mind, please sketch it out so we can see what you are thinking.
: compatible with Arduino UNO, 24V operation, all I/O's fused and protected