Shadowblades like one-handed daggers, Witchcraft, Air, and Scoundrel skills. They like DEX, INT and SPD as their stats.
As with most hybrids, their strength is in their versatility, rather than power. It's probably a good idea to first focus more on INT and disabling spells, and then boost DEX and SPD up a bit to allow you to backstab. Get as many attributes as possible from your items.