Treyarch are hard at work to keep players interested in Call of Duty: Black Ops 4 post launch following a well received beta. One fruit of their labour should be full fledged day and night cycle in the game's battle royale mode - Blackout.
Information regarding day & night cycle implementation came straight from David Vonderhaar, as he stated that such a mechanic is "very possible" in Blackout. Treyarch will not be dead set on forcing it into the game though, as Vonderhaar said that it might not make the cut, if it turns out to be detrimental to game performance.
Battle royale games don't really have a habit of implementing such systems, but it could be that they just didn't manage to get one right yet. For example, H1Z1 attempted to have different weather conditions as well as the day and night settings, but ended up changing it so 19 out 20 games play out in a sunny afternoon.
Vonderhaar's statement during his i with Eurogamer sounded rather enthusiastic about such a feature, but he added that folks at Treyarch want to make sure that unlocking such a capability in the Blackout map will not impact the performance of the existing time of day.
This suggests that Treyarch are happy to experiment with features not found in other battle royale games, which could potentially mean that the necessary refreshment may be on its way to the genre. According to Vonderhaar, day and night cycle might already be around, as he stated that Treyarch "have that capability and it's when and how do we deploy that capability.
Despite not straight up promising mountains of features in Blackout, including the day & night cycle, Vonderhaar pretty much confirmed it would actually come to the game at some point. Don't hold your breath for the feature launching together with the game though, as the context from the interview implies that the feature might still be a long way out from deployment.