That depends on the shinki. Sometimes it's just a matter of trying to protect each other. If say, you're stuck fighting an ayakashi with somebody who isn't your shinki, sometimes a contract is formed.

[There's a flash of surprise.] They didn't talk to you about that? No.. I guess they were busy with the festival...

Blight is what happens if you touch an ayakashi with your bare hands. It can also happen when a shinki feelings strong negative emotions. As their god, you feel that feedback.