Hexagram 02, line 6

龍戰于野.其血玄黃.

Zhan 戰, ‘battle’ (different from dou 鬥, ‘fight’). A battle of dragons was a bad omen. In the Lun Heng 論衡 it is said,

夏將衰也，二龍戰於庭…
When the downfall of the Hsia dynasty was imminent, two dragons fought together in the court…

Wen Yiduo 聞一多 links the fighting dragons to two passages in de Zuo Zhuan (闻一多全集, Vol. 10, p. 229):

鄭大水，龍鬥於時門之外洧淵…
‘There were great floods in Zheng; and [some] dragons fought in the pool of Wei, outside the Shi gate…
Legge, p. 675

內蛇與外蛇鬥於鄭南門中，內蛇死，六年而厲公入，公聞之，問於申繻曰，猶有妖乎，對曰，人之所忌，其氣燄以取之，妖由人興也，人無釁焉，妖不自作，人棄常，則妖興，故有妖.
Two serpents, one inside and one outside, had fought together in the southern gate of the capital, till the inside one was killed. It was six years after this when duke Li entered. The duke [of Lu] heard of the circumstance, and asked Shen Xu, saying, “Has Tu’s restoration come from that supernatural appearance?” The answer was, “When men are full of fear, their breath, as it were, blazes up, and brings such things. Monsters and monstrous events take their rise from men. If men afford no cause for them, they do not arise of themselves. When men abandon the constant course of virtue, then monstrosities appear. Therefore it is that there are monsters and monstrous events.”
Legge, p. 92

Ye 野, ‘borderlands’, the region far away from the capital where the civilisation and authorities are located.

Xuan huang 玄黃: ‘dark and yellow’. Wen Yiduo thinks xuan huang refers to different kinds of red, stating that since huang stems from the character guang 光 (‘to emit light’), related to fire, huang refers to the kind of orange  colour that fire emits. In the Shijing a fox ‘s robe is described as huanghuang 黃黃, ‘yellow-yellow’, but since a fox is not yellow Wen Yiduo states it must refer to a kind of red colour. In another poem it is said 莫赤匪狐, ‘Nothing red is seen but foxes’, so surely 黃 must also refer to some kind of red. Xuan 玄 should also be a red colour, as a commentary to the Shijing says 玄, 黑而有赤也, ‘xuan is black with red’, in other words a dark-red colour. David Pankenier agrees wholeheartedly with this:

“Wen Yiduo (…) shows that translating xuan huang as “black and yellow” is mistaken. Xuan is dark red, bordering on black, the colour of old coagulated blood, while huang (colour of the loess soil, present-day “yellow”) can shade all the way from cream coloured into brown.”
David Pankenier, Astrology and Cosmology in Early China, p. 54 n. 26

Rutt, however, finds this doubtful (Rutt, Zhouyi, p. 296). In the Shou Ci 守志 song from the Chi Ci 楚辭 collection of poems the expression xuan huang refers to one of the five celestial deities:

謁玄黃兮納贄…
I called on the Lord of Heaven and offered him tribute…
David Hawkes (tr.), The Songs of the South, p. 318

Hong Xingzu 洪興祖 (1090-1155) says that it refers to the chief authority if the five deities (“玄黃, 中央之帝也”).

When the expression xuan huang refers to colours it is used most often to denote the colours black and yellow, not ‘dark yellow’. The expression is also used in the Shijing to denote exhaustion, or sickness:

陟彼崔嵬, 我馬虺隤.
(…)
陟彼高岡, 我馬玄黃.
(…)
陟彼砠矣, 我馬瘏矣.
I was ascending that rock-covered height,
But my horses were too tired to breast it.
(…)
I was ascending that lofty ridge,
But my horses turned of a dark and yellow.
(…)
I was ascending that flat-topped height,
But my horses became quite disabled.
(tr. James Legge, adjusted)

Gao Heng thinks that xuan huang should be read as xuan huang 泫潢, ‘dripping and flowing’, so the sentence would become ‘their blood dripping and flowing’. This meaning would also fit the context in the sentence from the Shijing, 我馬玄黃, ‘my horse is dripping and flowing (sweat)’, but since xuan huang is an expression that occurs regularly in several old texts I doubt we should read it like this. The poem from the Shijing shows that with animals the colours refer to a physical weakened condition, which also fits this line from the Yijing.

Dragons battling in the borderlands. Their blood is dark and yellow.