Yes, its true that Mark has some problems with his eyes that's why he chose blue as the color for Facebook and when it comes to the meaning of the color blue, it is always connected with with depth and stability. -- That's maybe why he also chose this color when we talk about Business he wants stability. Who doesn't want stability with their Business right?
Blue is also important to the mind and body as it relaxes me and also calms me personally. It symbolizes a calming effect and maybe Mark also wants his targeted audience feel this way when they use Facebook.