Using the current tip of Tk core-8-6-branch, I also now see the problem of the extra blank (not necessarily black) window appearing (also it looks like a blank window at least sometimes briefly appears and disappears when IDLE launches).  This problem hasn't been seen with the release tarballs for 8.6.8 or 8.6.9.1.  But there are other, serious problems seen with the current tip of Tk core-8-6-branch.  See the discussion in Issue35485 for more information and for followup discussion.  For the moment, I'm going to close this issue as a duplicate of Issue35485 so we don't have the discussion spread out over multiple issues.  If necessary, we can update and/or re-open this one.