Warning: This article contains SPOILERS for Arrow season 7 episode 13, "Star City Slayers."
Arrow finally reveals the real identity of Katherine McNamara's Blackstar character, who's appeared in a handful of Arrow season 7's flash-forward sequences thus far; as it turns out, Blackstar's name is Mia Smoak, and she's the daughter of Felicity Smoak (and Oliver Queen?). Arrow was originally envisioned as a five-season series, which is why its flashback sequences only lasted five years. But since those scenes were integral to moving the show forward, Arrow's producers have opted to create flash-forward scenes.
Taking place in Star City in the year 2038, Arrow season 7's flash-forward sequences have primarily focused on Oliver's son, William Clayton/Queen, as well as the remaining heroes of Team Arrow attempting to uncover the mystery behind Felicity's murder, which surprisingly includes a plot to blow up the Glades. One of Felicity's contacts happened to be McNamara's Blackstar, whose first name was revealed to be Mia in Arrow's 150th episode. But now, Mia/Blackstar's last name has also been unveiled, thus confirming who her lineage.
In Arrow season 7 episode 13, "Star City Slayers," Dinah, William, Roy, and Zoe went to the former Team Arrow bunker and were ambushed by Mia and Connor Hawke, who reappeared in Arrow's 150th episode. While Mia was questioning William about what his goals were, he said that he just wanted to find out what happened to Felicity as she's his (step) mother. That revelation shocked Mia, as it meant the two were technically related. And at the end of "Star City Slayers," it's revealed that Mia is really Mia Smoak - Felicity's daughter.
Interestingly, Mia Smoak's identity wasn't the only big revelation from the flash-forward sequences in the "Star City Slayers" episode; Connor Hawke also found out that he's John Diggle's son, aka John Diggle Jr. This could very well be the impetus for Arrow recreating Legends of Tomorrow's Star City 2046, in which Connor becomes the new Green Arrow. But that's something that could continue to develop alongside Mia Smoak's storyline in Arrow season 7 (and possibly into Arrow season 8).
All in all, it's unclear what Arrow's writers and producers are trying to accomplish with Mia Smoak, as the character doesn't necessarily have a comic book counterpart. McNamara's Blackstar isn't based on any particular iteration of DC's Blackstar characters, and Mia Smoak doesn't have much in common with DC's second Speedy character, Mia Dearden. But it's certainly possible that Arrow is trying to set up a second Team Arrow, but in the future - with Connor Hawke becoming Green Arrow and Mia Smoak becoming Speedy. We'll just have to wait and see, though.