There are several attack types on Wotv:
Just like how it was written, that Magic Attack Up only work on Magic type attack. Note that Taunting Blade is Slashing Attack with MAG as its source of damage. At the end of day it was a slash attack.
Magic reflex works against spellblade attacks but mag atk doesn't affect spellblade skills?
No. Mag Atk is different from Mag. Mag Atk is compared to Slash, Pierce, Strike and Missile Atk. Mag is compared to Atk.
Only Slash Up and MAG Up will give benefits for spellblade. That's why Aigaion esper is the best for Howlet, Ramza and Mariele.
No, spellblade do slash attack but scale with MAG stats