I believe that doesn't actually work in CSGO (possibly it works in local servers against bots, but not in multiplayer where people actually want to play).

Hence people making workarounds such as changing movement keys to both move and to execute the command that hides any already created blood (and bullets):

https://steamcommunity.com/sharedfiles/filedetails/?id=23635...

or

https://wewatch.gg/blog/how-to-remove-blood-in-cs-go-binds-f...