Marines Power Armor may sport a specialized visor and optical upgrades, but the system that is not infallible. Blindness is among the most common Ailment to plague the Marines.
The most common source of blindness are the infamous Blindlings. They are spawned by Tartarus on lower difficulties, as well as upon the beast's death. However, far more common are roving hordes of Blindlings commonly known as "Bane Trains." They are so fast an nimble, they can catch a lone player around a corner or simply overwhelm him. Infestors that lay eggs can produce Blindlings. In all of these instances the thick viscus goo and intestinal matter left behind after the creature's death completely coats the Marine's visor until he can clear it.
The mighty Erebos can blind entire teams of players. A powerful psionic surge overwhelms the Marines senses, causing a long period of total blindness. Fortunately he only posses this ability on Normal Mode or higher.
Although uncommon, Ghouls can occasionally blind a player.
In all cases, the simplest cure for blindness is simply to wait it out. Blindness doesn't stack, so one must wait only 10 seconds after the last blind was inflicted for it to heal. Unfortunately standing still is an open invite to get hit. Move and kite, even Knife to try and escape any nearby enemies. Alternatively, running to Allied Units with their shared sight is almost as good as being cured.
Although probably a waste of energy, the Commando's skill Combat Hardened will instantly cure any ailment including blindness. However given the short duration of the ailment, unless its an emergency, it's better just to wait.
Any kind of flares, specifically the Road Flares will illuminate the area around you. It is important for characters with flare type skills to use them whenever allies have been blinded.