VS Code Python + Black formatter arguments - python.formatting.blackArgs

The examples of formatter-specific settings show the following:

"python.formatting.autopep8Args": ["--max-line-length", "120", "--experimental"],
"python.formatting.yapfArgs": ["--style", "{based_on_style: chromium, indent_width: 20}"]

So try:

"python.formatting.blackArgs": ["--line-length", "80"]

The issue is that you need =80 instead of 80 after --line-length for version 1.38.1 and above:

--line-length=80