ImportError: No module named 'flask.ext' [duplicate]
The "flask.ext" style of naming/importing modules has been deprecated for a number of years now. You should use the first style you described instead.
As for the suggestion that you install your flask packages globally, this somewhat defeats the purpose of using a venv in the first place. It makes it impossible to use pip freeze --local > requirements.txt to only save relevant packages, opening you up to package version conflicts.