Nature, if I am not mistaken, you are refering to one of those pics that you take at the beach, trying to capture the lights from the ship with someone in front but end up only getting the person correctly exposed with a very big black piece of cloth.
If this is it, then it is because your camera was set too fast to capture the scenery.
With flash photography, especially manual external flash (not one of those dedicated ones) shutter speed plays very little by way of getting the subject that is within the flash range correctly exposed. Whether you set the shutter speed from 1/60 to holding the shutter open in bulb mode, your subject will still be correctly exposed (provided your aperture to subject distance is correct per the table on the flash). This is because the speed of the flash is too fast for the shutter speed to make any difference, usually about 1/20,000 - 1/30,000.
But as you continue to extend your shutter exposure, you'll start to notice something. . .your background starts to get brighter and brighter. That's because long after the flash has extinguished, your shutter is still kept open to let the dim natural light in the distant to continue to expose your film or ccd if you are taking with digital.
As a simple rule, in such situations, you usually set the aperture to suit the subject to flash distance and then set the shutter to correctly expose the background.
But a word of caution, if you are not taking in complete darkness and your subject is not standing still, there will be some ghosting effect around him/her.