Yes. The iris is a barrier with an expandable hole in the middle through which light passes to the retina. Its function depends on blocking all light that doesn't enter the hole. All iris pigments leak a little light, called "straylight, " causing glare and mildly decreasing visual acuity. Blue pigment happens to leak the most light. Of more consequence is blue-eyed individual's skin photosensitivity.
Not necessarily. A blue iris is really a brown iris that has less brown pigment. This allows more light to travel through the iris, and in turn creates the blue color that we see. You would think that if more light is able to go through a blue iris (like light going through a lace curtain rather than a blackout shade) then blue-eyed people would be more sensitive, but in reality they are no more sensitive.
More light gets in. The color of the iris reflects the amount of pigment. The darker one's eye, the more pigment in their iris and thus the higher ability to block out incoming light. Blue irides have little to no pigment, allowing higher amounts of light to enter the eye, causing more glare. Blue-eyed baseball players are known to have lower batting averages as a result of this phenomenon.