All camera exposures have a component of ambient light. With TTL the flash is not always the overpowering power house the flash bulbs and strobes used to be.
See by Russ MacDonald. Look at the post about metering.
In basic task, set the ISO to a low value, shoot in the S, or A modes and use a shutter speed 1/125 to 1/200 this should force the ambient exposure to little to no contribution in the total exposure. You can use the M mode only but make sure the light meter is showing an under exposure.
If you are shooting in a mixed lighting situation, florescent or sodium vapor lights, you wand the ambient exposure to be minimal or you need to filter your flash and adjust WB to match the lighting.