To unlock Mason (Icon Character) in Black Ops 4 Blackout, you need to play as Reznov, find the “Stained White T-Shirt”, wake the sleeper agent and complete the match in the required place. If you fail to complete the character mission for any reason in one match, you need to find the Stained White T-Shirt again in the next game again.
The T-Shirt mission item can be found in Supply Stashes (Not Supply Drops)
HOW TO UNLOCK REZNOV (Required for Mason)
To Unlock Mason, you need to have Reznov first. His mission item can be found either in Estates or in Asylum.
★★★UPDATE (Nov. 21, 2018): You no longer have to play as Reznov to unlock Mason
HOW TO GET SUPPLY STASHES
A good strategy is to grab a helicopter at the start of the game and fly to the wind towers in the southeast. You can peak into wind towers from the air and check for supply stashes. The Towers have a decent chance to have 1-2 stashes each match.
Another strategy is to directly land at array where the next quest step is anyway. If you go through all buildings, you should also find several supply stashes.
HOW TO WAKE THE SLEEPER AGENT
The Sleeper Agent is basically just the machine with the green light from one of the Dark Ops Challenges. It is inside the central radio tower on the first floor.
After you completed all steps and completed the game with the required placement you unlock Mason.