It does seem that Logic 10.8 changed some internal behavior which broke plug-ins from multiple manufacturers. It seemingly affects plugins with both a standard and MIDI-enabled version.

As a workaround, the following steps seem to enable “Blackhole” to show up in insert slots, but of course only work if you are not using “Blackhole (No MIDI)” in your sessions:

1. Close Logic Pro
2. Navigate to Mac HD/Library/Audio/Plug-Ins/Components/, and move Blackhole No MIDI.component to your Desktop.
3. [Edit 11/8/2023] Restart your computer
4. Open Logic Pro
5. “Blackhole” should be available in the insert menus

We will continue to update this thread as we have more information.