How would one best represent the shadow skin killers of the dark eldar the Mandrake. Just use the rules for cameleoline cloak, blur field, or the mutation "shadow kin" in RT pg 369?

Mandrakes don't just hide in shadows they seem to be able to move through them. All those abilities are fine to represent their stealth but you'll need some way to represent them travelling through shadows, basically a teleport psychic power that requires a shadow as a catalyst. They may also have natural weapons ( I seem to remember them being rather toothy).

Hellebore

During the playtest, I did a rough draft of some Mandrakes and used them in a session. Rather than create a brand new ability to represent their unnatural sneakiness, I gave them the Phase trait, and a few appropriate talents like Assassin's Strike. It might not be a literal representation of their abilities (we don't know for certain), but from the perspective of how they move and fight, it's very effective. Thanks to Phase, they can appear from nowhere, giving no warning of their arrival, making them extremely effective ambushers. Assassin's Strike (and decent stealth and dodge skills) keeps them from getting pinned down in a fight and gives them the opportunity to escape again after an attack.