There are actually several different ways you can build your spellblade. The main difference is whether to use charge weapon or Blue Flame Blade.
Blue Flame Blade is more for crit focused thief builds and requires soulbinding to kitsune.
For my build, I main Chrysanthemum Petal/Wing of Asira and offhand Kunai
Focus Agility, Cunning, Willpower
Blue Flame Blade, Venom Strike, Whirlwind, Flames Within
Whiteblossom Ring
Eye for Weakness/Attrition Expert
Basically, you permacrit, gain obscurement every turn, and stack insane dot damage.