If I were trying to make a shadow blade, I would use dex and speed for primary attributes, and enough intelligence to cast the utility spells from the chosen spell schools which I think are air and witchcraft. For example I'm sure you'd want to pick up invisibility and the air spell that gives you physical resistance.
Because of the available utility you would have, it might be possible to play this combination with something like the Raistlin trait which would free up points from speed for intelligence. You might want some decent initiative though, which would still mean putting points in speed.
Guerilla increases your damage a lot if you're sneaking, it's a must have but I think it's a default for shadowblade. It doesn't seem to work with invisibility or way of the shadows but you can sneak from anywhere if you have either one activated. You'll definitely want the trait that allows you to run while sneaking.
If your backstab doesn't miss, the enemy you stabbed won't turn to face you, which means you can restealth with sneak. If you have someone distracting the enemies while you stay at their backs, you can backstab then restealth with impunity.
I had been thinking about if this class were viable ever since I first saw it in the character creation but I haven't tried it yet. Instead I played a dex/speed rogue with warrior skills so I could use spears, daggers, and bows; depending on the situation. Might try shadowblade next.
