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Your camera’s light meter reads the amount of light bouncing off the objects in your scene back into the lens and tries to get an exposure for 18% grey. So what if you shoot something black or white, yep you got it they will look a little grey if you don’t use some exposure compensation.
Here is an example shot in aperture priority mode with no exposure compensation. Of course we know the door is black but its not a nice deep black.
Here is the same door shot with -1 EV.