"Seir" here is another name for Edom, a country to the southeast of ancient Judah which acquired southern Judean territory during the Babylonian captivity and became known as Idumea. "These are the descendants of Esau the father of the Edomites in the hill country of Seir." (Gen. 36:9, RSV) "Dumah" is also a symbolic name for Edom. This "oracle" stands on its own as a unit, but is perhaps related to the previous oracle (Isa. 21:1-10) concerning the fall of Babylon. "Fallen, fallen is Babylon; and all the images of her gods he has shattered to the ground." (vs. 9, RSV)
The Abingdon Bible Commentary (1) states regarding Isaiah 21:11-12 -
Dumah was a place name, but is here only a play on the name of Edom. . . . From Seir there comes a cry, whether imaginative or real, asking for news about the future. The Babylonians have oppressed Edom. What hope is there? Will there come a better morning? The answer is intentionally ambiguous. The morning comes. Aye, and so does the night. The questioner receives no assurance, but is told to come again and inquire later. Perhaps he will get a more definite answer.
Dummelow's commentary (2) states that this is
An oracle concerning Edom, here symbolically called Dumah ('silence'), because of the silence and desolation in store for it.
Jameison, Fausset and Brown (3) relate
A PROPHECY TO THE IDUMEANS, WHO TAUNTED THE AFFLICTED JEWS IN THE BABYLONISH CAPTIVITY. One out of Seir asks, What of the night? Is there a hope of the dawn of deliverance? Isaiah replies, The morning is beginning to dawn (to us); but night is also coming (to you). Cf. Psalm 137:7. The Hebrew captives would be delivered and taunting Edom punished. . . .
Watchman - the prophet (ch. 62:6; Jer. 6:17), so called, because, like a watchman on the lookout from a tower he announces future events which he sees in prophetic vision (Hab. 2:1, 2).
what of the night - What tidings have you to give us to the state of the night? Rather, "What remains of the night?" How much of it is past? . . .
Reply of the prophet, The morning (prosperity) cometh, and (soon after follows) the night (adversity). Though you, Idumeans, may have a gleam of prosperity, it will soon be followed by adversity again.
Notice the ambiguity as to whether the "morning" was for Israel and the coming "night" for Edom (Idumea), or whether Edom would experience both "morning" and "night." Morning signifies prosperity and night adversity. It is a good reminder that, as long as the present order of the world continues, no matter how many "mornings" arrive, another "night" follows.
(1) Edited by Eiselen, Lewis and Downey. Doubleday & Company, Inc.: Garden City, NY. 1929.
(2) The One Volume Bible Commentary edited by J.R. Dummelow. MacMillan Publishing Co., Inc.: New York, NY. 1908, 1909.
(3) Commentary Practical and Explanatory on the Whole Bible. Originally published 1871.