Concise Bible Dictionary:

Under the figure of a bride the remnant of Israel says, I am “black,” describing herself as having become dark or swarthy by the rays of the sun; the scorching effect of affliction (Song of Sol. 1:5-65I am black, but comely, O ye daughters of Jerusalem, as the tents of Kedar, as the curtains of Solomon. 6Look not upon me, because I am black, because the sun hath looked upon me: my mother's children were angry with me; they made me the keeper of the vineyards; but mine own vineyard have I not kept. (Song of Solomon 1:5‑6)): “burning instead of beauty” (Isa. 3:2424And it shall come to pass, that instead of sweet smell there shall be stink; and instead of a girdle a rent; and instead of well set hair baldness; and instead of a stomacher a girding of sackcloth; and burning instead of beauty. (Isaiah 3:24)). The kingdom of the Medes and Persians is described as a chariot with “black” horses (Zech. 6:2, 62In the first chariot were red horses; and in the second chariot black horses; (Zechariah 6:2)
6The black horses which are therein go forth into the north country; and the white go forth after them; and the grisled go forth toward the south country. (Zechariah 6:6)
); and in the Revelation, in the third seal a rider on a “black” horse betokens scarcity (Rev. 6:55And when he had opened the third seal, I heard the third beast say, Come and see. And I beheld, and lo a black horse; and he that sat on him had a pair of balances in his hand. (Revelation 6:5)). It is symbolical of what is dismal and threatening.