Call of Duty: Black Ops 4 has several multiplayer modes and the battle royale one, Blackout, will be on free trial for a week, starting on 17 January 2019. Treyarch and Activision announced that the event will conclude on 24 January 2019.
Treyarch didn't specify the way players will get to download and boot up the free trial, but it's highly likely Battle.net launcher will simply get an additional icon on the sidebar that will allow the mode to be downloaded. PlayStation 4 and Xbox One users will get instructions on how to activate the trial from their respective stores.
There are no pre-loads happening, but it shouldn't be an issue since the trial will last seven days in total. Bear in mind that you will not be able to play other modes, since this free trial applies only to Blackout. This could hint at Treyarch and Activision testing the grounds to see whether a free to play model could benefit the battle royale mode.
It's still speculation at this point, but there is already a way to purchase a $40 version of Call of Duty: Black Ops 4 that only includes Blackout and Multiplayer. On top of that PC-exclusive deal, Battlefield V's battle royale is coming in March 2019, so a more accessible Blackout sounds like a good idea at this point.
Treyarch also rolled out an update alongside the 115 Day event that doubles experience, nebulum plasma and weapon experience. The update itself fixes the looting interface in Blackout, which should come as a major relief for Xbox One and PlayStation 4 players, who have been requesting the change for a while now.
Some other rumours indicate that Black Ops 4, and by extent - Blackout, is losing players due to Activision's monetisation meddling that turned players away and even made some influencers of the game. It remains to be seen whether the free trial and potential future incentives will be able to breathe some life back into Black Ops 4.