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. 1:5-6): “burning instead of beauty” (Isa. 3:24). The kingdom of the Medes and Persians is described as a chariot with “black” horses (Zech. 6:2, 6); and in the Revelation, in the third seal a rider on a “black” horse betokens scarcity (Rev. 6:5). It is symbolical of what is dismal and threatening.