I have to disagree here and say that I went BLU/RDM from 30 - 43 (until I finally got Mandibular Bite) and found it to be a nice option. Let's be honest, /NIN is for DMG mitigation only at low levels, you aren't gaining any real speed advantage. There really isn't a nice single hit spell to SA on /THF early on either. /RDM gave me more MP, enspells, and something I've not seen mentioned: the level 30 belt (Warlocks? idr) that gives +5 sword skill. The latent on that is active with RDM sub, and imho is worth subbing RDM for in that range.
At 44 I was having decent success with Mandibular Bite and SA, so I switched at that point to /THF.
I did it this way:
Level 1-29: BLU/DRK, /WAR or /WHM depending on my PT role.
Level 30-43: BLU/RDM
Level 44-73: /THF. Especially when you hit 60, /THF really shines for spike DD.
Level 74-Endgame: /NIN. Most of your spells that you use a lot (Fren Rip, Diss) are multi-hitters and don't benefit much from SA. I still use /THF for cannonball setup, but that's it.
Just an alternative suggestion to try
