Blessings were gifted to mistwraiths first by the Lord Ruler and later by themselves, in order to give them sentience and thus turn them into kandra. A blessing comes in the form of two hemalurgic spikes, one in each shoulder. The four blessings were: potency, awareness, stability, and presence. Coming from hemalurgy, the spikes were, of course, made with the death of a human.
The first generation of kandra were the packmen who accompanied Alendi and Rashek on their trek to the well of ascension. When Rashek took the power at the well he, fearing that someone else would get the twin blessings of feruchemy and allomancy as he had, he turned all living feruchemists into mistwraiths, granting sentience via blessings to his fellows. Every century the Lord Ruler supplied the kandra with new blessings so they could create a new generation of kandra. Blessings, being hemalurgic, allowed Ruin to take control of kandra, however due to their instilled instincts, most committed simultaneous suicide to prevent that from happening.
The blessing of potency grants a kandra increased strength and versatility. More or less the same effects as burning pewter would have. The kandra who have these are most often used in guard or similar duties in the kandra homeland. OreSeur possessed the blessing of potency until it was stolen from him by TenSoon.
The blessing of awareness grants a kandra exceptional use of his senses. The same or similar effects as tin has for an allomancer. These are the most common 'spy kandra' though the spies are in no means limited to those with this blessing. TenSoon possessed the blessing of awareness.
The blessing of stability grants emotional stability.
The blessing of presence grants greater mental capacity.