How should I build a +Spd, -Hp Blue Olwen?

She does make a better use of a Blade tome than her Dire Thunder, yes. Do not give her Life and Death as her Defense is already low enough as it is.

My opinion: do not use Olwen, her stats are pitiful. You probably have a 4* Reinhardt somewhere in your barracks. If you haven't done it already, promote him and use him.

Nice.

Anyways her best build is almost certainly Blade/Fury/Desperation. Her attack is so low by default that even quading can fail to net kills. Blade meanwhile can easily patch that fault up. Compared to a similar boon/bane Catria(the one who out stats her) she would have 1 less speed, 2 less hp and 6 less attack. In turn she has 2 more defense and 7 more Res. So effectively they have the same physical bulk. But Olwen handles mage hits better.

Now what does this mean? Effectively the two play out similar and let me tell you Catria with Life and Death can NOT safely activate desperation. She needs fury not only to chip her hp down for her, but to give her the bulk to safely take one hit. The good news is that Olwens extra magic bulk both means she can more safely enter desperation, but also she can use ploys to help drop a foe's res or attack making it easier to net kills/survive a counter.

Or basically yes Catria can best her as a bladetome mage, but Olwen ain't shabby at it.

Dire thunder is a bit of a joke on her unless you have a +ATK Olwen. And even then Rein almost always runs +ATK so she is 5 attack lower. Easier to get quads, but Rein is generally better.

For general use, the blade tome. You have the perfect IV for it. She's actually quite fast, which is perfect for the blade tome.

In AA, Dire Thunder is actually usable against low-res units, which can be useful. The example I keep coming back to is DC wrath Ayra, because most blue mages can't take a wrath RA so even swordbreaker becomes irrelevant. But if you double-tap her even with like 45 atk blue magic, she dies.