Added slave flashes make digital picture darker?

Using a Sony- DC F-828,I bought some slave flashes and plugged them in...When I shot a subjuect with my built-in flash, the slaves fired and my photo came out DARK. Turning the slaves off I get a nice photo but I thought the slaves would make it brighter if anything. How can I change my shot set-up to use the slaves correctly?

Does the camera do a fre-flash to test exposure?
Do you have red-eye reduction turned on?

If so, then the slaves may be triggering off the first flash, before the camera takes a picture.
So it sets the exposure for the full intensity of all flashguns - then only the on-camera fires for the exposure.

It may help others to know what make/model of slav flash you bought.

Post a photo with and without the flash to show us what you're talking about. I'm willing to bet it's completely normal and caused by a low amount of ambient light and/or using the camera in a mode where the shutter wants to be faster than when not using the flash.

An example will really help here.

I usually use slave flashes in Auto mode, not TTL. Another option is manual but it would take a flash meter or a lot of expermentation (could be possible with non-people shots). Manual/flash meter is going to take a little getting used to but with experience it will always give you what you want (that's the theory anyway! ;)).