The biggest reasons I can see for as to why blu is wanted over a number of other DD options is that they are low maintenance for the party, and they have a lower skill floor to be okay.

Blu has a lot of tools it can use so the support/healers don't have to worry about them as much, as well as help cover some other things for the party. If having to cast Haste individually, blu should be self hasting saving one target. Blus have a number of quick stuns to help prevent nasty tp moves or aoe spells. And for those that do get through, they often keep themselves up allowing better focus on the tank/other DDs. With winds of promy, they deal with most of the annoying non-na statuses. So overall it helps free up the demands on supports and healers. Hence the low maintenance.

As to the low skill floor, it really does not take much to get blu to a useful state. Set a handful of key spells for traits, and self buffs, buy items to unlock Cdc, and then add in decent gear. Not much is needed beyond that, and the spell farming weeds out some of the really terrible ones. Some of the other DDs take a bit more investment of time and gear to do alright. Pretty sure blu can compete with any DD, problem is takes a lot more work and investment to reach those points and be noticed. In a way its like comparing pld and ninja back in the day. Pld didn't take a lot of work to be an effective tank, while ninja took a nice bit more so it was much easier to differentiate between good/bad ninjas than it was plds.

On Cdc, its actually in a nice skillchain position, the problem is that its the same spot as many of the other prefered ones on the darkness side, which is the same issue with savage blade for light. Blu actually has plenty of SC options, thanks to their physical spells, though not sure how many are still useful for that as haven't messed around with chain affinity recently. So really if a blu can't alter what they use for skillchains they are just being lazy. (Also of note, they should know spirits within and sanguine blade don't have sc properties when not able to add to normal chain of party)

Honestly, much of this stuff has likely been brought about because the ws spam era towards end of aht urgahn and abysea, and the much easier ability to cap out attack delay and other key stats. As just the other night in a cp party, using a simple three step double light sc with 3 people using ws's it wasn't uncommon for me to have 2500+tp. And I'm only just on the edge of being able to self sc off my own weaponskills reliably, so while Cdc is nice its almost better for me to gear for savage blade or expacion first for party situations. Im willing to use whichever ws is best for the party, least sword has options unlike when leveled drg back in the day and used double thrust for close to 65 levels to open distortion.