A blue line Glock is a Glock packages to be sold to law enforcement personnel. A Glock dealer is selected to participate in the program and then stocks a certain amount of these “blue line” guns.
The Glock is not any different from a non blue line gun, but it did used to carry an extra magazine in the package. Now that all Glocks come with 3 magazines that no longer applies.
Blue line guns are only the popular Law Enforcement duty models, so basically full size and compact 9mm, .40 and .45 calibers. You won’t find a long slide ported 10mm in the blue line.
To me the biggest seller of the blue line gun is that if you present identification showing employment as a police officer, correction officer, or armed security you can get your gun a little cheaper.
The trade off is that quantities are limited, and so is selection.
I have never purchases a blue label glock because I like the OD green models and since they are not available in this package I tend to get mine in other ways.
A large dealer in south eastern TN (Craigs firearms) has a deal where if you get a letter from the head of your law enforcement agency stating you have public safety duties you can buy one gun a year at contract pricing. The gun does need to be a duty type gun so it is limited to things like ARs, Shotguns, and duty handguns. This is normally how I get mine.