C. F. Keil and Franz Delitzsch on 2 Samuel 18:21

2Sa 18:21

Joab therefore entrusted the Cushite with the duty of conveying to David the announcement of what had occurred. It cannot be decided with certainty whether הכּוּשׁי or Cushi is the proper name of an Israelite, or whether it signifies the “Cushite,” i.e., a descendant of Cush. The form of the name rather favours the latter view, in which case it would suggest the idea of a Moorish slave in the service of Joab.