Because /war offers you so much more than anything DNC has and the def drop from Berserk is mitigated with MG and Cocoon.
(I am not being combative here)
What exactly does /WAR offer that is "so much more"? Basically Berserk for dps or Defender if you are pulling for Omen swart farming. Other than that, it's really small. BLU main can already cripple defense of an enemy and Nature's Meditation without ever needing to pop Berserk, and in a zerg you are only getting 10% DA and slightly higher stats across the board since you might be at attack cap anyways. Now with that said, /war is still better than other options (lol), but that's just because BLU covers a lot of areas already that a sub would normally supplement whatever gap the job has. BLU doesn't have many.
Reasons to /dnc and why BLU main doesn't need it:
-Dual Wield - BLU can already set it, and the tier from /DNC is only II. You would have to make up more in your melee set to compensate (not a huge problem)
-Self Heals - BLU can already do significantly better/faster heals infinitely than Waltzes, and it's not tied to TP
-Violent Flourish - BLU gets way better option in Sudden Lunge
-Box Step - BLU gets Sweeping Gouge, Tenebral Crush, or Bligestorm/Tourbillion (if you want to use UL), which blows steps away for defense down. Yes I know they stack, but it takes a long time to apply the full bonus, and then you are tied to keeping it active, which is a further dps loss.
-Reverse flourish - Unnecessary, BLU can get TP way faster than using steps (dps loss) and activating this JA
Healing Waltz - This is the only reason to sub DNC on BLU and the uses are limited due to Windys of Promy. I used to use /dnc for Dawn HTBF on VD when I was soloing my body. Trusts would get hit with silence aoe for 1 minute and I had no way to remove it (and I couldn't survive that long without their heals), so healing waltz was the best way to help me beat the fight. It's also good for stuff like petrified or whatever on trusts, since those are things Winds cannot remove. But if it's for removing something on yourself, its not necessary since you can use Remedy/Holy Water/Panacea just the same.
To OP: I would argue that sub choice is not really significant for a job like BLU, as it is for say WAR or DRK, who rely onthat /sam for JA haste. It's probably one of the better jobs suited for things like Odyssey Sheol Gaol, where you are not really hindered at all from lack of sub. If you want to sub DNC, go for it, but it offers nothing that BLU can't cover on it's own. As was stated, /dnc is a convenience factor of using sneak/invis, or freeing up spells to cover your utility if you need it.