How do I import multiple .py files to my main.py file

Question

I have a folder structure like this:

C: 
|_Blueprint
│   main.py
│
└───toolbox
        blueprint_tools.py

When I run this code in Blender's scripting text page:

from toolbox.blueprint_tools import dimension, array, modify

I get an error in the console

Traceback (most recent call last):
  File "\main.py", line 20, in <module>
ModuleNotFoundError: No module named 'toolbox'
Error: Python script failed, check the message in the system console

---

I swear, when I run this code (on other projects) outside of Blender this structure and code runs fine. But I get an error when running this script in Blender for testing out my Add-On.

If I compile the Blueprint folder into a .zip and install it everything works fine.... ?? I'm lost, could anyone please help me.

If I run the code like this: (added a . before toolbox for CWD)

from .toolbox.blueprint_tools import dimension, array, modify

Traceback (most recent call last):
  File "\main.py", line 20, in <module>
ImportError: attempted relative import with no known parent package
Error: Python script failed, check the message in the system console