To unlock Seraph in Black Ops 4 Blackout, you need to open the secret box at Firing Range, get the Annihilator, defeat 1 enemy with it and reach the placement goal. If you fail to complete the character mission for any reason in one match, you need to find the Juneau’s Tags again in the next game again.
Unlocking Seraph + Annihilator
The Annihilator can be found in a box inside the green building on the west side of the firing range. This box does not spawn each match but it does appear there a lot more frequently than the other character mission items.
Once the Special Box spawns, grab a handgun and shoot the white target (only appears when the box is here). The box will not open with any other weapon type.
Grab the Annihilator, take out one enemy and make it to the placement goal that can be found if you check the Annihilator in the menu.