Definitely the media - I've never had any dye spots appear on any other blank media that I've bought.
I've had some MIT Verbatim 16x DVD-R (MCC 03RG20), which showed similar characteristics after burning with my Benq 1650.
After completing a burn, I noticed a big spot on the underside of one of the discs. I'm sure I posted it on this forum somewhere. I was told that this problem is due to defective media.
However, after updating my firmware, the dye spots seemed to have disappeared since I started burning more of these discs (in the same burner of course).