Is the feature Heart of Darkness available for customisation of background in AL (following the guidance of pg 125 of the PHB)? Probably it is equivalent to ask if the background "Haunted One" is part of the "sample backgrounds". This would be strange since in that way one could obtain another background like "Haunted One" but with 2 tool proficiency/languages (instead of a total of one).
When creating a custom background, you follow the rules in the Player's Handbook (this is addressed on page 6 of the FAQ_7.1.
To customize a background, you can replace one feature with any other one, choose any two skills, and choose a total of two tool proficiencies or languages from the sample backgrounds. You can either use the equipment package from your background or spend coins on gear as described in chapter 5.
This works for any eligible background feature (Heart of Darkness included). However, it is worth noting that the Haunted One background has an errata (also repeated in the same page of the FAQ):
The errata is summarized as follows.
Skill Proficiencies. Choose two skills from among Arcana, Investigation, Religion, and Survival.
Starting Gold. This background does not include starting gold.