I made a rather large list of enemies from which BLU could learn useful spells (not listing spells with Death, Encumberance and/or auras because it would never happen):
Bosses or boss-like:
Where?: Dynamis-Xarcabard
What?:
1) Oblivion Smash (AoE damage) - Unbridled Learning
2) Transfusion (AoE Drain) - Regular spell
3) Mana Storm (AoE Aspir) - Regular spell
Where?: Limbus or Abyssea-Uluguerand
What?:
1) Guided Missile (AoE damage) - Regular spell
2) Discharger (Magic shield effect + Shock spikes) - Regular spell
Where?: Limbus
What?:
1) Chemical Bomb (Conal Elegy and Slow II effects) - Unbridled Learning
2) Nuclear Waste (AoE elemental resistance reduction) - Unbridled Learning
Where?: WKR or Delve.
What?:
1) Static prison (ligntning damage + Paralyze + Shock + Dispel) - Unbridled Learning
2) Reverberating Cry (Lightning damage + Magic Defense down + Magic Evasion Down) - Regular spell
Where?: WKR or Delve.
What?:
1) Firefly Fandango (AoE Paralyze, Flash, Max MP down) - Unbridled Learning
2) Canopierce (AoE damage + Rasp) - Regular spell
3) Potted Plant (AoE damage + Bind and Slow) - Regular spell
Where?: WKR or Delve.
What?:
1) Glassy Nova (AoE ice damage + full dispel + stats down) - Unbridled Learning
2) Polar Roar (AoE ice damage) - Regular spell
Where?: Kamihr Drifts
What?:
1) Calcifying Claw (Single target Petrification + knockback) - Unbridled Learning
2) Divesting Stampede (AoE Magic def down + defense down + knockback) - Regular spell
Where?: Voidwatch (Aydeewa Subterrane)
What?:
1) Atropine Spore (Single target damage + sleep + silence + paralyze) - Unbridled Learning
2) Night Stalker (AoE damage + TP reset) - Regular spell {long recast}
Where?: Voidwatch (Buburimu Peninsula)
What?:
1) Chymous Reek (AoE damage + Stun + Shock) - Unbridled Learning
Where?: Voidwatch (Valkurm Dunes)
What?:
1) Diluvial Wake (Conal water damage + stats down + defense down + magic defense down) - Unbridled Learning
2) Bolt of Perdition (AoE lightning-based damage + knockback + Silence (mute) + Amnesia) - Unbridled Learning
Where?: Voidwatch (Bibiki Bay)
What?:
1) Deep Sea Dirge (Amnesiaga) - Unbridled Learning
2) Blowhole Blast (AoE damage + paralyze) - Regular spell
3) Echolocation (AoE damage + silence) - Regular spell
Where?: Walk of Echoes (#7)
What?:
1) Dark Star (AoE dark damage + Defense down + Magic defense down) - Unbridled Learning
Where?: Walk of Echoes (#13)
What?:
1) Shrieking Gale (AoE damage + full dispel) - Unbridled Learning
Where?: Walk of Echoes (#11)
What?:
1) Yaksha Bliss (Conal damage + attack down + magic attack down) - Regular spell
Where?: Abyssea Tahrongi, Misareaux, Attohwa, Vunkerl, Grauberg, Altepa as well as Walk of Echoes (#15)
What?:
1) Diabolic Claw (Single target 3x physical attack + Magic Defense down) - Regular spell
2) Hellish Crescendo (Paralyzga) - Regular spell
3) Stygian Sphere (Heals HP) - Regular spell
Where?: Al'Taieu
What?:
1) Vitriolic Barrage (AoE damage + poison) - Regular spell
2) Primal Drill (AoE damage + Bind) - Regular spell
Where?: Abyssea Grauberg, Attohwa, Vunkerl, Altepa, Misareaux; as well as Voidwatch and Walk of Echoes (#12)
What?:
1) Scapula Beam (AoE damage) - Regular spell
2) Arm Cannon (AoE fire damage) - Regular spell
Where?: Abyssea Grauberg, Altepa as well as Nyzul Isle.
What?:
1) Homing Missile (Severe damage) - Unbridled Learning
2) Discharge (AoE lightning-based damage + Paralyze) - Regular spell
Where?: Abyssea Uleguerand
What?:
1) Scintillant Lance (Conal light-based magic damage + Amnesia) - Unbridled Learning
2) Rampant Stance (AoE damage + Stun + Bind + Aspir) - Unbridled Learning
3) Damsel Memento (Heals HP + Erase) - Regular Spell
Where?: Several [S] areas (good hunting!)
What?:
1) Wrath of Zeus (AoE lightning-based damage + Silence) - Unbridled Learning
2) Acheron Kick (Conal physical damage) - Unbridled Learning
Where?: Meeble Burrows
What?:
1) Sticky Situation (AoE magical damage + Plague + Slow + Silence) - Unbridled Learning
2) Flesh Syphon (Single target physical damage + absorb attributes) - Regular spell {long recast}
Where?: Several [S] areas as well as Abyssea Tahrongi and Attohwa
What?:
1) Extreme Purgation (AoE status-effect draining move; both negative and positive) - Unbridled Learning
2) Aeolian Void (Conal damage + Silence + Blind) - Regular Spell
Where?: Outer Ra'Kaznar
What?:
No idea which ws's they do. Anyone got a list?
Regular enemies:
Where?: Kamihr Drifts
What?:
1) Zealous Snort (Haste + Magic Defense+ + Counter) - Unbridled Learning
2) Sweeping Gouge (Fan-shaped physical attack + Defense down) - Regular spell
Where?: Outer Ra'Kaznar
What?:
I never fought one; and I can't find weaponskills online. {Please assist}
Where?: Multiple Adoulin areas
What?:
1) Empyreal Glister (AoE light damage + Defense down + Magic defense down + reset TP) - Regular spell
Where?: Multiple Adoulin areas
What?:
1) Phototropic Blessing (Heals HP + Regen + Defense+ + Magic Defense+) - Unbridled Learning
2) Phototropic Wrath (Attack+ + Magic attack+ + Haste + Enlight) - Unbridled Learning
3) Consecration (Single target physical damage + stun) - Regular spell
Where?: Ceizak Battlegrounds (and other areas)
What?:
1) Phlegm Expulsion (AoE Slow + Silence + Amnesia) - Unbridled Learning
2) Slicing Sickle (Single target physical damage + defense down) - Regular spell
3) Raptorial Claw (Single target physical damage + magic defense down) - Regular spell
Where?: Yorcia Weald
What?:
1) Stink Bomb (Magical damage + Blind + Paralyze) - Regular spell
2) Tickling Tendrils (Physical damage + stun) - Regular spell
Where?: Marjami Ravine
What?:
1) Molting Plumage (AoE wind-based damage + Dispel) - Regular spell
2) Pentapeck (Physical damage + Amnesia) - Regular spell {long recast}
Where?: Cirdas Caverns
What?:
1) Enervating Grasp (AoE damage + Paralyze + Bind + Defense down) - Unbridled Learning
2) Orogenic Storm (AoE damage + Silence + Slow + Blind + Evasion down) - Unbridled Learning
3) Metamorphic Blast (Conal attack + sleep) - Regular spell
NOPE! None of those; instead we get this:
4) Subduction (AoE weight) - Regular spell -.-;
Where?: Nyzul Isle and Abyssea-Uluguerand
What?:
1) Artificial Gravity (AoE damage + Gravity) - Regular spell
2) Restoral (heals HP) - Regular spell
Where?: Abyssea-Uluguerand (and several other areas)
What?:
1) Dire Straight (Single target damage) - Regular spell
2) Earthshatter (AoE damage) - Regular spell
Where?: Multiple [S] areas as well as Abyssea and Voidwatch
What?:
1) Summer Breeze (Regain) - Unbridled Learning
2) Autumn Breeze (Heals HP) - Regular spell
3) Cyclonic Turmoil (AoE wind-based damage + knockback + dispel) - Regular spell
Where?: Abyssea-Misareaux or Uluguerand, as well as Sky and other areas
What?:
1) Electrocharge (Single target thunder-based damage) - Regular spell
Where?: Meeble Burrows
What?:
Thrashing Assault (Single target 4x hits damage + Defense down) - Regular spell
Where?: Abyssea-Tahrongi
What?:
1) Corpse Breath (Conal darkness-based damage + Blind) - Regular spell
Where?: Abyssea-Misareaux, Ceizak and other areas
What?:
1) Leeching Current (AoE water-based damage and Drain) - Unbridled Learning
2) Hypnic Lamp (light-based Sleepga) - Regular spell
Where?: Mount Zhayolm and other places
What?:
1) Erratic Flutter (Haste) - Regular spell