Slimline won't play blue disc
That sounds like a lens problem. The PS2 blue discs are read differently than the regular ones, and I am told the blue discs give out more information that the regualr ones, so your system is not able to recieve it. There are basically two solutions..
1). Get a "high quality" lens cleaner disc. The cheap ones can damage your system's lens.
2). Get a can of air that people use to clean out keyboards. You want to make sure you can see the lens and blow the dust off of it. Just blowing air inside the system will not work.