They attempt to address this question in Shiny happy people ::
15 EXT. GARDEN AT HOTEL / ANGEL INVESTIGATIONS - NIGHT
The woman walks outside to find Angel leaning against a railing, staring at the ground.
WOMAN
That fragrance... It's lovely. What is it?
ANGEL
Must be the jasmine.
WOMAN
Oh, night blooming. So beautiful, so lonely...like you.
ANGEL
(looks up at her, then away) I shamed you.
WOMAN
No.
ANGEL
I did. I almost killed that man. I think I would have if—
WOMAN
Angel, you were only trying to protect me. There can be no shame in that.
ANGEL
I don't want to fail you again.
WOMAN
Then don't.
ANGEL
It's not that simple.
WOMAN
Why isn't it?
ANGEL
'Cause...what I'm feeling.
WOMAN
You must let go of your anger towards that poor soul.
ANGEL
No, not anger...happiness. (woman smiles) I'm so happy that you're here.
WOMAN
And that's a problem?
ANGEL
For me, yes. (looks away) If I get too happy, if I allow myself to feel what I'm feeling, (nearly in tears) Angelus might—
WOMAN
I know. But soon none of that will matter. Evil will be banished by the deeds we will do, Angel. Even the evil that's inside of you. That too will be gone, and all that will be left is the beauty.
ANGEL
Eradicate all evil.
WOMAN
Yes.
ANGEL
Is that even possible? We've been fighting for so long. (shakes his head)
WOMAN
I know. I've seen it all. It's why I've come back. (tilts Angel's head up) You are my general. I have faith in you.
ANGEL
You... have faith in me?
WOMAN
(looks up to the patio where the others have gathered) I have faith in all my warriors, each and every one of you. You are all up to the task.
I think he doesn’t turn evil because her “ pure bliss” is a form of mind control and domination. She’s a rogue higher power, it’s possible her ability to manipulate and control others is able override the curse because it’s not legitimate happiness. Once Her blood gets in his system and her spell is broken , his happiness is shattered, because it was only an illusion.