Problem most people have is that there are SO many people who say "I have BLU" and are absolutely crap at it. BLU is one of the hardest jobs to play in FFXI due to it having elements from all the skill area's. Your DD elements require you to be familiar with the damage formula and know how to min/max each ability and adapt to each situation. Know when to deploy support functions, when not to and how to customize your spell load out for that particular event. Most people who play MNK just look at what another MNK is doing and copy it, that is the extent of their efforts and it produces a serviceable DD. If a player does that on BLU they'll fail horribly as evidenced by all the horrible BLU's you see running around the server.
That all being said, once you get a reputation for knowing your sh!t, people will be far more open to bringing you along. BLU brings a ton of utility without sacrificing much DPS. There is a whole host of offensive, defensive and utility abilities that a BLU can deploy in a fight to make everyone else's job easier. It requires having in-depth knowledge of the game and good judgement, so it's not something I recommend people take up without first having experience in other jobs.