Lasers have been part of weapons systems for over 30 years, dating back at least as far as the first use of laser guided bombs late in the Vietnam war. Troops have also used low powered lasers for rangefinders and target designators for a long time. To the best of my knowledge, the power levels of these devices are low enough that they won't cause permanent eye damage. In addition, those devices usually use light frequencies that can't been seen to avoid detection, although as you pointed out, invisibility of a laser beam doesn't necessarily mean the laser is eye safe.

High powered lasers are a different ballgame. If portable, they could be used for antiaircraft, antimissile, and antiartillery defensive purposes. They'd probably also be effective against soft skinned vehicles (e.g. trucks and Humvees). It'd be a while before they'd be effective against thick skinned vehicles like ships and tanks, but with enough power, even those can be vulnerable.

If lasers are used as weapons, it's true that some people will be blinded even by low powered units. However, people are blinded on battlefields by all sorts of weapons from firearms and mines through artillery and bombs. Just because someone can be blinded by a weapon, it doesn't automatically mean that it's outlawed by the Geneva Convention. You can blind someont with just about anything. However, if the lasers were deliberately aimed at an enemy's eyes with the intention to blind, then you can make a case for saying that would be prohibited.