BlackList

How do I view / edit items I have added to my Blacklist?

Without having more information only thing to say is in the same place as you set them.

https://www.avast.com/faq.php

Presumably you mean the web shield, site blocking.

If so:
AvastUI > Settings > Active Protection > Web Shield - Customize > Site Blocking.

No, I want to view the Blacklist for email received with Microsoft Outlook through the Avast plug-in mail shield.

There is no blacklist in the mail shield.

In Outlook I have the Avast Antispam plugin installed. When I receive an unwanted email I click the Blacklist button and the email address is stored somewhere. I’ve looked in Outlook and it is not stored in the Standard Outlook list. I have the standard Outlook filtering disabled. So, the Avast anti spam filter stores this data somewhere. I would like to know where and how to access it. Clearly, for my investigation, it is handled by Avast.

Found it. Go to Settings / Active Protection / Customize Anti-Span / Blacklist.
I wish there were sort options on the list.