Why does adding an external flash make my manual-exposure photo darker?

When using an external flash as a slave, the built in flash can be set to only trigger the slave and not fire with the shutter timing. The slave flash has an IR sensor that receives the signal sent by the master flash before the shutter opens and follows the instruction to fire once the shutter is open (part of the instruction is exactly how long to wait before firing).

If you're using it as a slave flash, and is being triggered by the built in flash and the slave is in front of the camera pointing towards the subject, it's quite possible it's not getting triggered at all (due to not receiving the command from the built-in flash).

If you turned the flash towards the camera, but away from the subject, it's more likely that the light was hitting the slave and setting it off.

Have you noticed that the external flash is actually firing or are you just looking through the view finder and only recognizing the built in flash going off or are you using Live View?

To correct this use the swivel feature of the 430EX to allow you to turn the bottom half of the flash with the receiving unit towards your camera and still point the flash head at your subject. You may also need to use a small reflector in front of the built-in flash to bounce the signal towards the 430EX if it is placed at an angle outside the cone of light emitted by the built-in flash.