it would be neat if the recovery time gradually decreased in the last few hours so that during the last hour it was only a couple of seconds, but you could only place on blank spaces.

That would cause too much chaos and encourage botting imo

I guess when you say black space, you mean one that's white because it's never been edited, as opposed to one that's white because someone painted it that way? That would have some interesting effects, like that you could preemptively defend bits of a design that aren't background.

I think that ultimately that would be a bad idea for the same reason why it's good - trolls/bots could quickly add in the usual bilge before anyone could react and the final image would have to either be edited or would be tainted.

I also think that this time it wasn't well enough publicised, whereas next time there'll be more players and so won't be much blank space anyway.