The four blessings are: Awareness (2 tin spikes), Potency (2 iron spikes), Presence (2 copper spikes), and Stability (2 zinc spikes).
Kandra are actually mistwraiths gifted with a pair of spikes- both of the same metal, stealing a human attribute. They are not sentient without the spikes. TenSoon took OreSeur's as well, giving him the blessing of potency in addition to the blessing of presence he already had. (2 Iron Spikes granting physical strength like a constant pewter burn, 2 copper spikes granting eidetic memory)
Whether or not they could receive a benefit from allomantic of feruchemical spikes has never been explicitly stated, which is why I posed the question.
I don't really see why they wouldn't be affected, so long as the spikes were correctly placed, so until we get a Word of God, I'm going to assume it's possible and throw a rogue Kandra Serial Killer at my party of heroes. MUAHAHAHAHAHAHAHAHAHAHHA! This is going to be fun.