Blue flies into the sky, so the sky is blue.

蓝色飞入天宇，所以天空是蓝的。

youdao

Why is the grass green and the sky blue?

为什么草是绿的，而天是蓝的？

youdao

In spring, the sky is very blue, the cloud is white, and the air is fresh.

春天，天空很蓝，云是白的，空气是新鲜的。

youdao

The painting is like a bright blue sky with floating circular shapes on its edges.

VOA: special.2010.06.23

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

But if you hit one of those beautiful winter days when the sea is calm and the sky is blue, continue along the coast path for about two miles until it curves into the hidden bay and tiny old harbour of Lamorna Cove.

BBC: Britain's winter wanderland

The sky is blue, the air is crisp and the leaves are aflame with gold, crimson and orange.

BBC: In Boston, leaf peeping by bike

This refraction eliminates both the red end of the spectrum (red light is not bent enough by the air layer to form part of the second image) and the blue frequencies (which are scattered away: this selective scattering is why the sky is blue).

ECONOMIST: In a flash