Oh boy so yeah I didn't take into consideration that you can premade a group that I forgot to add and overlooked it my bad however, what if the person is brand new and chose BLU as their leveling class since Yoshi P said you can play it with just ARR but the person has no friends. That means the person can't play the way they want to by grouping with people and letting the game naturally take their course so they get a feel of the class in a group setting since the player won't know about PF or anything unless they researched the class. Not to mention the trust system all the way through leveling, again is not in a player party it's NPCs and yourself with 0 player interaction, I have no issue with solo play at all I really don't but when you don't actively seek interaction and don't have premades then how are you supposed to get dungeons done? That's an issue.
Another issue is how BLU levels it's open world grinding mobs and FATEs which is nothing unique since many people are doing Eureka grinding artificial EXP for their Elvl for relics and such so already this is nothing new and innovating it just creates more emphasis on the Trust system which is more grinding and hand holding which we have enough in this game. If a player levels like that the whole way, then finally gets to endgame as a BLU then the endgame content for them they can't do since they're regulated to just BLU trials and premades with friends or FC mates if they have them which doesn't fly the player didn't need to do that until now so it's a huge slap in the face.
^ this is another great point here who beat me to the punch what if I want to play BLU? I can't. I don't have that choice I have to play other classes because BLU isn't available at this time to go through the MSQ so even if the player does level BLU you can't do the newest things thus making you level an alt or another class that you don't enjoy who has a different playstyle. It's bad design you should be able to play as any class as you want despite the content it shouldn't be a "limited job" they said BLU is OP but why make something so OP that you refuse to balance around current content? They can scale the abilities around the class instead of making it this Godlike class that can do anything but nothing at the same time.
This may be an experiment but already I'm dreading it giving players the lack of direction only to sidewind them at the end saying "you can't play the newest content please play another class" is so bad it's like if I played a Death Knight in WoW and all I could do was level with my undead NPCs and only kill mobs it's terrible. SE keeps putting more and more limitations on their game and it's going in a bad direction where that freedom of choice for players is taken from them when they have a carrot on the stick which is BLU only to be told they can't play with non-limited classes.
Edited as you posted that since you can't start off as BLU.