A curious Cloud surprised the Sky,
'Twas like a sheet with Horns;
The sheet was Blue —
The Antlers Gray —
It almost touched the Lawns.
So low it leaned — then statelier drew —
And trailed like robes away,
A Queen adown a satin aisle
Had not the majesty.
F509 (1863) J1710
Like many people, Emily Dickinson enjoyed cloud watching. We've seen them as ships, sailors, and pirates in earlier poems as the poet reverses sea and sky. In this poem, however, she draws an amusing sketch of a big flat cloud, low to the ground, that looked like a sheet with antlers. In the second stanza it gains some dignity, drawing back in a stately fashion to trail away. No longer looking like a sheet, the cloud now appears to be beautiful robes, more majestic than any queen's.
The poem is in common ballad or hymn form: iambic tetrameter alternating with iambic trimeter. Dickinson divides the third line of the first quatrain into two lines to make a nice emphasis on the contrasting colors, blue and gray.
It's odd to have a blue cloud, but when the cloud is thin and the sky intense, they can look very blue. Perhaps its color is what "surprised the Sky."