Saelon was caught by Borlas many years before the The New Shadow tale is set when he was stealing an apple in Borlas' garden. Borlas' anger and description of Saelon's behaviour as doing orc's work struck in to Saelon's heart - he felt deeply agrieved and held this as a deep grudge.
Some years later, Borlas and Saelon are sat in the garden across from Minas Tirith. After Borlas describes the Dark Tree, it is Saelon who later mentions that there are men who have grown under the shadow in recent times and whispers the name Herumor to Borlas (the possible leader of a rebellious faction in Gondor).
Saelon leaves to return to Borlas under the cover of darkness, when he will clad in black. He did not state where he was then to head but left the invitation for Borlas if he too is robbed in black.