Is blockland's single player even rated by the ESRB? I could of sworn Badspot says he hates the ESRB.
Yes he does hate the ESRB ( The pic you posted).
However the Blood and gore mod is enabled by default even in singleplayer gamemodes, not just multiplayer.
ESRB doesn't rate online interactions for this reason
Included gamemodes can be played in singleplayer.
Anyways lets point our attention back to the main issue that is OP's request.
Anyways, I have come up with an idea on how to fix OP's problem.
We would need to update the default Blood and gore mod to support client sided preferences, to explain, basically for the Blood and gore addon you would also need the Blood and gore client, and that would add a toggle in settings for Blood on or off, when you join a game that has it enabled the addon would check your preference and simply toggle the effect, BUT there is a problem, as far as I know it is not possible to stop rendering particle effects for specific clients only, I may be wrong on this, but I do not remember it being possible.
But that's a start at least, second problem being if Badspot would even consider pushing a community made update, I'm sure for bugs or exploits he would, but for a single add-on I don't think he would consider it necessary though.