The blue guns are actually for training purposes. They have the same weight as the weapon they are imitating and are often used in hand to hand or unarmed combat training to show how to disarm a person when they are wielding a weapon which actually has the heft and weight of the actual gun. Most times they're made out of a more solid rubber though. The solid blue color is to indicate their known working replica status.
Additionally, most conventions will allow you to use airsofts guns with your costume, provided the orange tip is intact, and they have been reviewed by staff to show the magazine is empty and rendered non-fireable, usually just with aq simple zip tie around the trigger. Just make sure to check with the convention rules of the con you are going to ahead of time.
