它（应该是前一句话里提到过的东西）被染黑的羊毛,已经被雨水冲得掉色了.
句子的主干是
Its wool had been washed clean by the rain.
which had been dyed black是定语从句,修饰wool
不过个人觉得had been washed clean by the rain似乎不应该用过去完成时.因为定语从句里面用得是过去完成时,表示过去的过去发生的事情,而主句再用过去完成时,逻辑上显得有点不对.这里可能用现在完成时has been washed更加合适.