I don't see how Hei can have a "nice conversation" with Yin seeing his intent on killing her. Mao said so himself that the Yin from the past and the current Yin are two different beings. Besides, we'll be getting 4-OVA episodes about Hei and Yin, so we don't really want this to turn into a romance series.
No series can cover everything up; some things are just better left in mystery.