BLU gets total crap for clubs, Shellbuster included (the DMG is very low, the delay is higher than the good swords, and it has no accuracy on it like the swords any good BLU would use). Even with the 2 merits I have in club from WHM, it doesn't make up for the fact that every club we get sucks horribly. It's kinda gay, considering we have the same club skill as WAR, yet WAR gets a bunch of nice maces and we don't. :/ Also, our club skill is several levels below our sword skill, and by 48 (the level where you could use Shellbuster, which sadly is the best club we get until Titan's Cudgel at 65) it's far enough below your sword skill that it kills your accuracy and attack, even without the crappy stats.
Honestly, I didn't have much trouble getting PTs until recently (like post-60). I've always listed all my available subs in my search comment, as others have said (and I have WAR, WHM, BLM, RDM, THF, and NIN leveled). As for NIN sub: I'd say that between the levels of 30-40 it's certainly fine. You can offhand a second Centurion/Bastokan/Republic Sword for extra accuracy. THF sub doesn't really shine until 40, when you get Chain Affinity. And even then, no spells are truly awesome for stacking with SA; your best spells are still multi-hit ones (Bludgeon and Jet Stream). Mandibular Bite at 44 is the first awesome spell for stacking with CASA. Personally, I subbed WHM til about 34, then BLM (just because I wanted Warp for when PTs were over; WHM would have been better). I subbed NIN occasionally from 30-40, then almost all the time from 40-60, with only a few PTs where I could tell I'd get away with /THF (it takes a lot of hate, and you don't have Utsusemi to take up the slack; you need a good tank so they can get hate back fast enough for you to not die). 60+, I haven't subbed anything but THF, of course.
As for what a good BLU does in a PT; DD, with a liberal serving of Head Butt. XD Stunning annoying mob abilities/spells helps a lot, and our physical spells are awesome DD; you just hafta make sure to conserve MP long enough to go without resting (Refresh is a must, obviously; and later, Sanction refresh and Auto-Refresh from setting the right spells), which will also keep you from taking hate too much and getting killed. For PTs without a reliable sleeper, you can sleep a link with Pinecone Bomb, and for PTs without a dispeller, you can use Geist Wall (so long as you're in an AoE-safe area; it has short range anyway). Magical spells are pretty much worthless for actually casting; either they won't stick without AF body, or they do crap damage. I'm sure that with BLM sub, HQ elemental staves, Magic Atk. Bonus gear/spells, and INT gear, a BLU could do decent damage with their magical spells, but it still wouldn't be as good as /THF or /NIN with physical spells and melee gear.
I think that one of the problems with BLUs is that a lot of players still don't understand them. BRDs (and even some CORs) get all pissy because they can't refresh them (since they melee, and thus get stuck with the melee songs/rolls), melees at the lower levels don't understand that they need MP to deal any decent damage and will pull when the BLU has no MP, and the crappier RDMs tend to forget to Refresh the BLU cuz they are just a melee in their eyes. Since BLUs kinda need a RDM to refresh them (for the above mentioned reason), PTs with only a BRD or COR to refresh will sometimes avoid them. I love having a BRD or COR in my PTs on BLU, though, so long as there is also a RDM to keep me refreshed. ^^
And again, as some people have said, there are a lot of crappy BLUs. I've PTed with a few of them myself. I remember one in the Boyahda Tree who was subbing WHM, using a Shellbuster, and only casting a bunch of really low level spells like Sprout Smack, Wild Oats, Bludgeon (which sucks by 50+), Poison Breath, etc... And our RDM was the only healer, in addition to having to Refresh like 5 out of the 6 people in the PT and dispel/enfeeble things, so we asked him to do some healing, since he always had a ton of MP and was subbing WHM. So what did he do? He started casting Healing Breeze every once in awhile (and usually right at the start of a fight, getting himself nearly killed before we could get hate off him, wasting more of the RDM's MP to cure him), and didn't touch Cure III. Still always had a ton of MP. I think I saw him use Mandibular Bite once or twice after he saw me using it. In a PT in Wajaom fighting Colibris, there was this really crappy JP BLU with the most horrible gear I'd ever seen for someone of this level. He even had two of those level 30 latent effect earrings that are activated by your subjob: the DRK one and the RNG one (or maybe it was BST...), and he was subbing NIN. He also used almost no spells that did any real damage; he kept spamming magical ones, especially AoE ones (which got us several links). He only self-SC'ed after he saw me do it (did the same SC, Red Lotus Blade -> Jet Stream for fusion) and then kept trying to MB Bomb Toss on it, even though he failed 9 times out of 10. I switched to using Red Lotus Blade -> Sickle Slash for Gravitation, hoping he'd copy me again and have no AoE spells to MB, but no luck there. <.<;
Anyway... this post has gotten out of control; I'm gonna stop it here. XD
Just do what I've always done on Bst (since people would never consider inviting a Bst) Just start one! You are a killer Blu 
