Contrarily to Nikon’s flash system, Canon’s doesn’t add the flash exposure compensation if it is set both on the camera and on the flash.
If you set the flash exposure compensation on the flash of -1EV and the of -0.5EV on the camera, the flash takes priority and the total exposure compensation is -1EV. The exposure compensation on the camera is ignored.
To contradict the previous statements: The Canon’s Flash Exposure Compensation is in addition to the fill-in flash automatic reduction of the flash output of around -1.5EV to -2EV.
If you dial -1EV on the flash, and it senses that this is a fill-in flash, and you have a Canon 7D, Aperture mode and dial-in a -2EV, the exposure compensation will be -3EV.