Just so this post doesn't appear unanswered, the issue was addressed in darthpotato94's other post:
http://www.digitalfaq.com/forum/medi...m-optical.html
To add some more comments addressed in this thread, and not the other...
And an "external" burner is really the same thing as an "internal burned in an enclosure", for the sake of semantics.
Dark spots can be storage/handling damage -- or more likely just bad media. I have a stack of cheap BD-R here that need to be photographed for our upcoming "Blank Blu-ray Quality Guide", and each of the discs has massive blotches or spots where the disc either did not burn, or did not burn well. The TFT tests look "fine", but the disc is obviously damaged in some way. It will cause the Blu-ray player to "burp" at that spot, too. (So remember that "scans" on computers are not always 100% infallible as some sites/people online would lead you to believe!) A good "test" is actually a whole battery of tests, and under controlled conditions.
Either way, always use the best burners and discs.