Hi. Just got back into Arma 3 after a long hiatus; re-learning ALiVE.

I'm having trouble with Blacklist Markers appearing on the map during the mission. I've named the variable space and have included that variable name in my Mil and Civ Placements modules' Blacklist fields. I've also named them BL_x, Blacklist_x, TAOR_BL_x and a bunch of other stuff, nothing works.

In the wikis, there is a the paragraph: Blacklisted areas will be ignored and can be defined using area markers with the name specified in the module params, for example BL_Faction_xx (e.g. BL_R_1). If TAOR and BL area markers overlap, Blacklist always takes precedence.
See the TAOR turtorial for a more detailed guide on how these work.

Specified names in module params... I have no idea what that means. Also, the TAOR Tutorial does not mention Blacklist Markers at all...

What have I got wrong?

If you have blacklist markers called marker_1, marker_2, and marker_3 (their variable name, as Friznit mentioned) then in the blacklist field, you would put exactly that - marker_1, marker_2, marker_3.

Also, make sure the marker you're using is either a rectangle or an ellipse, as I don't think the blacklist will recognize any other types.