If the game has a slider you are simply telling the engine where to stop producing brighter light than the display can actually see, the game will also often configure other aspects of the image to ensure that the image remains looking right within this range you have set.
The game will always send the raw image data which will tell the TV what to display, the metadata that is sent never changes. The Metadata contains information about what type of display was used to master the content on. Your display is then meant to be able to use this data to make some changes to the overall image to try ensure that the image remains look broadly similar, despite your TV perhaps not having the hardware capability of the Mastering display.
In the case of the B7, it's max output is around 800nits, so technically going above that won't actually gain anything. With a UHD movie, if you send that TV a disk that is sending metadata which says "the brightest pixel is 1000 nits in this movie" then the TV takes that information and will adjust the image displayed so that anything that the disc is saying should be 1000 nits, is actually moved down to the brightest the TV can do (800 nits). So every bit of data that exists between 800-1000 nits on your display needs to be mapped downwards, without impacting too much on the rest of the image.
This is the part that all HDR10 Tvs actually handle differently.
So in a game such as injustice 2, that HDR adjustment you are making should only be affecting the very brightest things, but it perhaps sounds like it has a very good lightning system and display mapper and as you are increasingly the slider up towards those achievable 3500 nit values, the algorithm used to generate the grading is seeing there is a bit more overhead, so not as much 'artificial' contrast is required, so you are seeing the blacks lighting up a little.
So in a nutshell , if your TV is configured in a way that means it isn't making dynamic adjustments or moving too far away from the HDR10 PQ standards, setting your in game setting to match your TV's peak brightness (which is somewhere between 700-800nits) should provide you with a technically optimal image for a game.