[ Disclaimer, --- , ]
The Blue server is currently (in October 2019) Project 1999's most popular server.
This is undoubtedly because it offers the closes experience to playing EverQuest as it was in its early years. Because of the massive number of changes made to live EQ, and because all other emulated EQ servers have a fraction of the population of Blue (and often allow multi-boxing, which further makes them feel empty), even players who played during Shadows of Luclin, Planes of Power, or later will find that Blue "feels more like home" than any alternative server.
Blue = Beta
However, what many players fail to understand is that, to the people behind Project 1999, letting everyone replay classic EverQuest on Blue is only a secondary goal. The primary purpose of Blue is actually to serve as a testing ground for the developer's true goal.
In many ways the Blue server can be thought of as being like a Beta Server for the Green Server. Thanks to Blue anything that the developers code is likely to be tested by hundreds, if not thousands of different players. However, precisely because it serves this purpose, Blue isn't a true realization of the Project's goal.
Beta Side Effects
For example, in 2000 the Kunark expansion finished after less than a year, but on Blue that era lasted for seven years. This gave the devs the time they needed to fix everything in Kunark and get Velious's code right, and they were only able to do both things because of Blue's "beta tester" population.
Blue After Green
Although it was originally created to serve a "beta" server for Green, Blue will remain open (at its current point in the timeline, just before Luclin) even after Green's release. It may receive "classically-inspired" custom content:
Development will not stop for Green and Blue. The project will continue to make adjustments to fix bugs and restore classic functionality as it becomes possible to do so. There may also be things that weren't properly 'era locked' when implemented that may need to be adjusted. We will also be considering additional content for Blue that may be custom in nature, but adhering to a classic feel.
Blue: Not 100% Classic
For example, unlike Green Blue will continue to allow custom UI skins and the pet window, even though both were added shortly after the classic era.