Huh. A quick look at my set up shows the Blockchair ads are indeed on my own personal filter list and not the pre-installed ones, which means I must have added them at some point, although I don't remember doing so. You can permanently add an ad (heh) to your block list with 4 clicks with uBlock Origin (uBlock Origin icon -> select element picker tool -> click on the ad -> click on "create"). I do this aggressively with any ad which gets through the standard filters, so I probably did it years ago the first time I ever saw them and then simply forgot they exist.