For the general issue of interpreting control chars, see #23220 (and duplicate #24572).  IDLE is a development environment, not a production environment.

If Tal's hypothesis is correct, a solution for Bernard would be a new addition I want for Run Customized: 'Run with System Terminal'.  The idea is that people who know the IDLE editor should be able to develop terminal-specific code in IDLE.  Instead of Save, open or switch to Terminal/Command Prompt, enter 'python -i path/to/file.py, switch back to editor, one could hit Shift-F5, click [ ] Run with Terminal, click [OK].  The use cases are use of functions like Windows-only kbhit() and interpretation of output chars as on the default system console/terminal. The resolution for this issue should be that I open an issue so that others can contribute to doing this.

As for what is happening with the keras output:  The block character appears to be the Inverse Bullet character.  Here are 1 and 3: '◘ ◘◘◘'. In tk Text on Windows, the height and width depends on the font.  In some, it is twice tall as wide, as at https://www.compart.com/en/unicode/U+25D8.

Here are 1 and 3 \b backspaces: ' '.  Firefox displays each as a square box with 0 0 0 8.  On Windows, tk displays \b and many other control chars as a White Vertical Rectangle, https://www.compart.com/en/unicode/U+25af.  Here are 1 and 3: '▯ ▯▯▯', except that when representing backspaces, there is not the wide space between.

In the png, the printable ascii runs overlap by 7 while the blob runs overlap by 6.  If the blobs were \bs, there would seem to be 1 too few.  Until Bernard copies and pastes the 'normal' output, or exactly describes it, if dynamic, and copies and pastes the 'wrong' tk output, I don't want to speculate further on what he saw.