What a quintessential example of the "intertextuality" of the scriptures. This passage would be an excellent teaching example of how the scripture "drops words" that connect the current discussion with the stories and imagery scattered all throughout the scriptures. This is how the scriptures are designed to be read, with one passage illuminating the other. Awesome, and a large reason I'm a believer today.
So to be clear, John is not simply describing people and events. The people and events are filled with symbolism and each symbol connects with history and other scriptural people and events such that the effects of the imagery are multiplied and feed off of each other. Praise God and give him glory.
Sackcloth is simply coarse cloth, such as one would use to make a sack such as a sack for potatoes or corn. It is rough to the touch in contrast to "fine" material. The word also refers to a garment made of it. It has been used throughout Jewish history as a symbol of mourning:
Term originally denoting a coarsely woven fabric, usually made of goat's hair. It afterward came to mean also a garment made from such cloth, which was chiefly worn as a token of mourning by the Israelites. It was furthermore a sign of submission (I Kings xx. 30 et seq.), and was occasionally worn by the Prophets.
[1Ki 20:31-32 KJV] 31 And his servants said unto him, Behold now, we have heard that the kings of the house of Israel [are] merciful kings: let us, I pray thee, put sackcloth on our loins, and ropes upon our heads, and go out to the king of Israel: peradventure he will save thy life. 32 So they girded sackcloth on their loins, and [put] ropes on their heads, and came to the king of Israel, and said, Thy servant Benhadad saith, I pray thee, let me live. And he said, [Is] he yet alive? he [is] my brother.
Notice that they do not say, "Let's get some sacks and cut out holes for our heads and arms..." but rather "let us, I pray thee, put sackcloth on our loins". This example of simply taking a piece of coarse fabric and tying it around one's waist seems to be what "putting on sackcloth" originally entailed. The fabric is scratchy and at least partially see-through so it is to place oneself in a humble circumstance:
[Mat 5:4 KJV] 4 Blessed [are] they that mourn: for they shall be comforted.
It might or might not be made of animal fur. The verse in the OP is not specifically said to be "of hair" so it was not necessarily of animal fur as is the sackcloth mentioned in an earlier chapter:
[Rev 6:12 KJV] 12 And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood;
So perhaps the material is not the focus. The material might tell us the level of discomfort and humility it provided but it seems that what the author wants to do is to associate the "sun" with "prophets and prophecy" and perhaps particularly Elijah and John the baptizer:
[2Ki 1:8 KJV] 8 And they answered him, [He was] an hairy man, and girt with a girdle of leather about his loins. And he said, It [is] Elijah the Tishbite.
[Mat 3:4 KJV] 4 And the same John had his raiment of camel's hair, and a leathern girdle about his loins; and his meat was locusts and wild honey.
The association of "rough clothing" with prophets and prophecy was so strong that false prophets would wear it to pretend to be a true prophet:
[Zec 13:4 KJV] 4 And it shall come to pass in that day, [that] the prophets shall be ashamed every one of his vision, when he hath prophesied; neither shall they wear a rough garment to deceive:
So when the 6th seal opens and the sun is shouting down "prophets and prophecy" in association with the "moon turned to blood" it seems that the prophets are raining down the prophecies of doom upon Babylon (IE: Jerusalem).
[Rev 11:3, 6 KJV] 3 And I will give [power] unto my two witnesses, and they shall prophesy a thousand two hundred [and] threescore days, clothed in sackcloth. ... 6 These have power to shut heaven, that it rain not in the days of their prophecy: and have power over waters to turn them to blood, and to smite the earth with all plagues, as often as they will.
SACKCLOTH (שַׂק, saq; σάκκος, sakkos). A rough cloth made from animal hair, usually that of a goat or camel. In both Greek and ancient Near Eastern literature, it refers to a type of material used in various household duties such as the construction of clothing and sacks; the word can also refer to the clothing or sack itself. In the ancient Near East, the use of a garment made of sackcloth came to symbolize sorrow or submission. The Hebrew and Greek words undoubtedly share a common etymology.
Use in Biblical and Related Literature
Sackcloth in the Bible can refer to a type of material (Lev 11:32; 2 Sam 21:10; 11QTb; Shepherd of Hermas, Similitudes 8.4), to sacks (Gen 42:25, 27; Josh 9:4), or to an item of clothing worn to express contrition (1 Kgs 20:31–32; Psa 35:13), lamentation (Jer 4:8), mourning (Gen 37:34; Psa 30:11), supplication (Josephus, Antiquities 12.300), or repentance (1 Kgs 21:27; Matt 11:21). When used as a sign of grief, it is sometimes accompanied with the use of ashes (Esth 4:1; Isa 58:5; Matt 11:21; Luke 10:13). In the Old Testament the wearing of sackcloth is also often accompanied by tearing one’s clothes (1 Maccabees 2:14; History of the Rechabites 8:2), pulling out one’s hair (Ezek 27:31; Amos 8:10), going barefoot (Josephus, Antiquities 8.362), lying or sitting on the floor (Esth 4:3; Josephus, Antiquities 19.349), wearing ropes around the head (1 Kgs 20:31–32; Josephus, Antiquities 8.385), and praying or fasting (Dan 9:3; Jonah 3:5). The practice of using sackcloth in the form of a robe to demonstrate penance continued into at least the early medieval period (Richter, Meaning, 182).
Usage as Clothing
Sackcloth was uncomfortable to wear as clothing. Wearing sackcloth, especially as an act of penance, was practiced by men, women, children, servants (of the penitent), and priests; religious items and possibly even farm animals could also be covered in sackcloth as part of a community’s act of penance (1 Kgs 19:2; Jonah 3:8; Judith 4:10). It is probable that someone could wear sackcloth in several different ways, depending on the period and the occasion—as a small body cloth (Gen 37:34; Jer 48:37), an undergarment (2 Kgs 6:30; Judith 9:1), an external cloth meant to be seen (Judith 10:3; Josephus, Antiquities 5.37), or as a larger covering (1 Chr 21:16). Sackcloth was unattractive and dark, if not black, in color (Rev 6:12; Isa 50:3; Josephus, Antiquities 7.154; Apocryphon of Ezekiel [compare 1 Clement 8:3]). It was also easily recognizable, and thus people could be prohibited from wearing it in certain places or situations (Esth 4:2). Sackcloth is contrasted with fine attire (Isa 3:24; Psalms of Solomon 2.20). Other types of material (such as burlap, ὠμόλινον, ōmolinon, Sirach 40:4) could possibly be substituted when needed.
Richter, Klemens. The Meaning of the Sacramental Symbols: Answers to Today’s Questions.
Translated by Linda M. Maloney. Collegeville, Minn.: Liturgical Press, 1990.
Estes, D. (2016). Sackcloth. In J. D. Barry, D. Bomar, D. R. Brown, R. Klippenstein, D. Mangum, C. Sinclair Wolcott, … W. Widder (Eds.), The Lexham Bible Dictionary. Bellingham, WA: Lexham Press.