Bobcat, some scholars view Darius the Mede as the uncle of Cyrus. As uncle and viceroy and/or vassal of Cyrus the Great, he would have been sidelined in the honor stakes by historians. Nevertheless, as you say, the question of his identity must remain open for the time being. Interestingly, Keil & Delitzsch in their Commentary had the following to say on Dan. 9:1:
Darius given kingship. In the word hâmelake the Hophal is to be noticed: rex constitutus, factus est. It shows that Darius did not become king over the Chaldean kingdom by virtue of a hereditary right to it, nor that he gained the kingdom by means of conquest, but that he received it, (qabeil, Dan. 6:1) from the conqueror of Babylon, Cyrus, the general of the army.
Goldingay mentions that it could refer to a title or throne name: “ Although Zech 1 links Jeremiah’s seventy-years prophecy with the events of 519, which are close to seventy years after the fall of Jerusalem in 587 b.c. , the Darius by whose reign chap. 9 is dated must be not Darius I (Porphyry) but the Darius of Median birth introduced in 6:1 [5:31]. His “being made king” of Babylon here may reflect his “acquiring/receiving” the kingship of Babylon there (and see Setting above). Ahashweros is the regular Biblical Hebrew equivalent of Old Persian Khshayarsha, Greek Ξ ερξης (Xerxes: so Old Greek here; Theodotion transliterates—cf. Revised Version); see Esther; Ezra 4:6. Historically, Darius I was the father of Xerxes I; the order of events in Ezra 4 might have suggested that Xerxes (v 6) preceded Darius (v 24). Khshayarsha, like Darayavaush itself, is probably a throne name, meaning “hero among rulers” (Frye, Heritage, 97), and Wiseman infers that it could thus have been borne by an earlier figure such as Darius the Mede’s father, though it seems a problem with this suggestion that Khshayarsha is a Persian name. In Esther, a Greek translation takes ahhswrws to refer to Artaxerxes, while in Tob 14:15, Ασυηρος (Asueros) denotes Uvakhshtra or Cyaxares the Median conqueror of Nineveh in 612 B.C. (Frye, Heritage, 72–73). He might be seen as Darius the Mede’s predecessor/ancestor/father, and ahhswrws is actually as close a transliteration of Uvakhshtra (Akkadian U-aksa-tar) as it is of Khshayarsha (see W. S. Auchincloss, “Darius the Median,” Biblioteca Sacra 66  536–38; Torrey, Journal of the American Oriental Society 66  7–8).” 
 Goldingay, J. E. (1998). Vol. 30: Daniel. Word Biblical Commentary (239). Dallas: Word, Incorporated, 239.