It's not possible to customize the Email Blurb for DocuSign Completed/Voided/Declined emails at runtime on a per-envelope basis by using the API. However, it is possible to change the contents of the emails that DocuSign sends for various events (i.e., Completed/Voided/Declined) by customizing the "Email Resource File". This guide contains information about how that's done: https://support.docusign.com/en/guides/docusign-email-resource-file-v1-1.
As the guide shows, a different template within the Email Resource file defines the email content for each email event. For example, the Envelope Declined template within the Email Resource File (described on page 30 of the guide) defines the structure/contents of the email that DocuSign sends to recipients when an Envelope is declined.
Keep in mind that a Resource file is associated with a "Brand" -- so if you have multiple (different) scnearios that you want to customize emails for, you could create multiple Brands and customize the Email Resource file for each, and then specify the desired Brand with each envelope you send (which will set email content for that envelope). Doing things like this would give you somewhat "runtime" control over email contents.
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