Thank you for your report. I am unable to reproduce that behavior.  I suspect that the most likely explanation is that you are not actually using the python.org 3.8.2 but rather an IDLE and python from some other distributor (like Homebrew) that has a different version of Tk.  There is an easy way to check that.  From a Terminal.app window, enter the following commands to the Unix shell:

    /usr/local/bin/python3.8 -c "import sys;print(sys.version)"

You should see:

    3.8.2 (v3.8.2:7b3ab5921f, Feb 24 2020, 17:52:18)
    [Clang 6.0 (clang-600.0.57)]

If not, you are not using Python 3.8.2 from python.org.
Otherwise, then try:

    /usr/local/bin/python3.8 -m idlelib

and see if the Preferences window is still black.  If so, another possibility is that your saved IDLE preferences may be interfering.  Choose Quit IDLE from the IDLE menu then try moving your existing IDLE preferences out of the way so that IDLE will use the "factory defaults":

    cd
    mv .idlerc old_idlerc
    /usr/local/bin/python3.8 -m idlelib