first off do you have the euro version of the game or the other version?
also you may want to check your video drivers are up to date and also the right version for your vid card
check also to make sure your shader detail is set to high and turn off grain effect see if that helps

check to see if a mod you have may be causing it to when you unsub a mod it will still be in your games folder you have to go to steam\steamapps\left4dead2\left4dead2 addons and delete the mods your are not using from there as well otherwise they sit there and mess with your other mods

personally i think they need to give left 4 dead 2 a dlss update

No I don't believe I have the euro version also everything is up to date as well, I tried deleting all the addons in the workshop folder just recently and I still have the same issue, whats even stranger is when I load up the TUMTaRA mod map to test out addons the blood is dark how it should be, I'm so mind boggled on how to fix it as I have no idea what would be causing it, given granted I do have 624 addons installed but no conflicts with the blood mod I'm using...