Blue discs are CD based PS2 games as opposed to DVD based games (silver backed). The most probable cause of your problem is a problem with your PS2's laser. Either the laser is going bad/breaking, or the alignment is off. There are tutorials online on how to adjust the laser, although I've never done it personally. Alternatively, I've seen laser assemblies sell for $30-50 or so and you could just swap it out.
It's possible that something else could be your problem, but this is the most likely cause.