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In poetry, the language is loaded with imagery. A cup holds things, as does the sky: clouds, rain, lightning, the sun etc. Deep blue would be describing the shade of blue the sky is reflecting from Earth's waters. Morning tends to be a pale blue while later in the day, the shade would deepen. Poets want their readers to see through the writer's eyes; so, they appeal to visual images. At times, they may use a sound imagery like onomatopoeia or alliteration, but this is not the case in the particular line you quoted.
Can this possibly refer to the Holy Grail?
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