This whole thing about checking private accounts by blocking them is straight out of the "good old" category, which used to work flawlessly. I also came across a discussion of this method at
https://www.imperiumpublication.com/forum/general-discussion/how-to-get-first-1000-followers and they recommended using follower trackers or activity analysis apps that don't violate platform rules. Basically, the old trick is a thing of the past. Only secure tools provide accurate data.