Now hold on a minute there, Mr. Writer. Blackwatch at the time was still affiliated with Overwatch though strictly as a Black Ops division. All Gabrial Reyes would have to do to get Mercy to participate is,
Gabe: I need Dr. Ziegler for a mission where her expertise would be useful.
Jack: Did you fill out the TRN-5R paper work?
Gabe: Yup.
Jack: Good to go!
Furthermore, Blackwatch around the time of its inception places Brigitte around the age of 15. Just throwing it out there.
Now, if you wanted to say the guys doing this to them are Talon, then we'd be in the right ballpark of current events and it'd fit in with what they want to do. If this were Blackwatch, it'd be an HR nightmare.