You have two independent metering systems in use with flash in so the camera will set its exposure to ambient and the flash tries to do the same. All is not lost however, putting the flash into TTL-BL mode alters the flash metering to consider subject illumination while the camera, in Matrix metering is considering the entire scene. TTL-BL is wonderfully flexible.
I use it primarily in M mode so I make a choice as to the ambient exposure of the entire scene, such as with strong back-lighting. In that case the camera is adjusted to expose the scene to how I wish the brighter background it exposed and leave the flash to do its magic, properly exposing the subject. If I want a darker background, I make settings for scene under exposure by the degree that fits my intent. Usually a couple stop underexposed to make sure the subject is isolated. In fact, TTL-BL is so versatile, I very seldom use TTL flash. Bounce, focused, back-lit, foreground lit, any ratio between background and subject light is handled very well.
Shooters of other brands of cameras probably are more envious of your flash system than anything else, Nikon really has it down. Stan St Petersburg Russia