Maybe it could refer to either the poet or the sun throughout the entire line, if you think of the mantle blue as being the actual sky that the sun is able to twitch over his shoulder.

或者两者皆可,从整句的行文看来，如果你们把蓝色的丝巾看作是真实的天空的话,那么太阳的确是能够在他的肩上轻轻颤动。

耶鲁公开课 - 弥尔顿课程节选

This really provides the central rhetorical base for Milton's Lycidas.

这为弥尔顿的《利西达斯》提供了行文基调。

耶鲁公开课 - 弥尔顿课程节选