Why do i see a dark section near the CD Drive ring, when looked from a tilted angle?

Those darker regions closer to the ring lock on CD Drive are an indication of how much of the CD Drive space is written with data. CD Drive has sectors and the rpm of the CD drive Optical Reader will read these sectors and show it into the file system of the operating system you are using. The dark section does not necessarily show how much is available space but can be an indicator of if the CD drive is empty