In “silver dew” the word “silver” is used in this sense:
silver, adj. 5.a. Having the whiteness or lustre of silver; silvery. Chiefly poetic.
Oxford English Dictionary.
So this describes the silvery appearance of the dewballs on the flowers, due to the way they reflect and refract light. “Silver dew” is in fact a long-standing cliché in English verse; I’ll give three examples, but more could be adduced.
Whilst pleasant Thyme shall labouring Bees invite
And Silver Dew† be Grashoppers repast,
John Ogilby (1654). The Works of Publius Virgilius Maro, p. 26. London: Thomas Warren.
† “Silver” here is original to Ogilby. It does not appear in Eclogues V.77 “dumque thymo pascentur apes, dum rore cicadae” (and while bees are fed by thyme, while cicadas by dew).
Whilst sick’ning Flow’rs drink up the Silver Dew,
And Beaus, for some Assembly, dress anew;
Samuel Garth (1714). The Dispensary, p. 13. London: Jacob Tonson.
and Cynthia† still doth steep
In silver dew his ever-drouping head,
Charles Gildon (1718). The Complete Art of Poetry, volume 2, p. 284. London: Charles Rivington.
† The moon. “Cynthia” was an epithet of the goddess Artemis, who was born on Mount Cynthus, and associated with the moon.
In “wash the dusk with silver”, the word “silver” could refer to the dew. This would be a figure of metonymy, referring to one thing by a closely associated thing. In this case, because the dew has a silvery appearance, the poet can use “silver” as a metonym. But an alternative reading of this line is that “silver” refers to the silvery light of the star, which we have just been told is “glimmering”. Both readings seem appropriate to the scene that Blake is depicting, so there is no need to pick one or the other.
There is no contradiction between “scatter” and “wash”, because we can (if we like) take “wash” in the following sense:
wash, v. II.10.a. Water-colour Painting. To cover with a broad layer of colour by a continuous movement of the brush.
Oxford English Dictionary.
If we take the word “wash” in this sense, we personify the evening star as a painter, metaphorically covering the canvas of the dusk with the evening dew, or with its rays of light, depending on how we take the word “silver”. William Blake was a painter in water-colours, so this image likely resonated with him.
To understand “sacred dew”, we need to recognize that the poem is in the form of a prayer or hymn to the evening star, which is being personified as a divine being, like the ancient Greek god Hesperus. If the evening dew were scattered by a god, it would be considered sacred.
Finally, why is “scatter thy silver dew” is in the form of a request, but “the fleeces of our flocks are covered with thy sacred dew” is in the form of a statement? I think this is because the poet intends us to imagine time passing and evening advancing through the course of the poem. Line 2 says, “Now, whilst the sun rests on the mountains”, so at this point it is just before sunset, and it is appropriate to request the evening star to scatter its dew, but lines 10–11 say, “Soon, full soon, does thou withdraw” so at this point the evening star itself is setting. By the time the evening star sets, the evening dew has already settled on the flowers and the sheep, so that the speaker’s request has already been answered.
