Farʿ can denote the hair of the head, while related vocabulary also evokes branching. Yazīnu is an active imperfect from z-y-n: “it adorns.” Al-matn is the back over which the long hair lies.
Fāḥim communicates intense blackness; athīth adds fullness and thickness. One describes color, the other volume and texture. Aswada belongs to this accumulating descriptive sequence and must be parsed with its governing context in view.
The poet uses a cumulative chain of attributes: position over the back, saturated blackness, and extraordinary density. Translating all three simply as “beautiful hair” would erase the distinct semantic work of each word.