Blackwatch is a thing of the past.
Overwatch’s elite unit to react more quickly to certain problems and which, among other things, led to the dissolution of Overwatch and the Petras Act,
Just because Overwatch is active again in “Zero Hour” (but the Petras Acts is still activ and so Overwacth is still illegal and at the same level as Talon) Blackwatch is not automatically reactivated.
Half of the Blackwatch troop is Talon, the other half are Overwatch members or gang leaders.
Even if I see the possibility for Reaper, Sombra and Widowmaker to join or at least to help Overwatch (with the latter I hope for such a (redemtion) Ark) I see it as very unlikely that Blackwatch will be active again.