It is not the flash that is causing the dark background, it is your ambient exposure. What you need to do is expose for the ambient light as if you are not using flash. The ISO you chose is very low and you want one that will give you a reasonable fast shutter as the ambient exposure will be part of your exposure. I like to do my primary exposure in manual and use auto flash though you can use auto exposure if you like. As you are shooting Nikon you can take advantage of the wonderful Nikon CLS feature called Balanced Light. This will require your setting your metering mode to Matrix so that the camera can meter the background and tell the flash to expose to put the same amount of flash on your subject as is on the background. Also enable High Speed Sync on your body to allow the use of flash at high shutter speeds. Set your flash for TTL-BL and you should be close to where you want to be if not spot on.
