If I (or a Herald rule) adds a blocking reviewer to a diff, the resulting email generates by this action contains the following text:
$USERNAME added a reviewer: <span class="">$REVIEWER
For example, see D17159#203634, which generated the following email:
joshuaspence added a reviewer: <span class="">epriestley</span>. REPOSITORY rARC Arcanist REVISION DETAIL https://secure.phabricator.com/D17159 EMAIL PREFERENCES https://secure.phabricator.com/settings/panel/emailpreferences/ To: joshuaspence, #blessed_reviewers, epriestley Cc: epriestley