Hi there! Here's how you can change the default border color in Excel using BBCode:

1. Open a new Excel workbook.
2. Click on the "File" tab in the top left corner of the screen.
3. Click on "Options" in the left-hand menu.
4. In the Excel Options window, click on "General" in the left-hand menu.
5. Scroll down to the "When creating new workbooks" section.
6. Click on the "Use this as the default font" dropdown menu and select the font you want to use.
7. Click on the "Use this as the default border color" dropdown menu and select the color you want to use (in your case, black).
8. Click "OK" to save your changes.

That's it! Now, whenever you create a new workbook, the default border color will be black. If you want to change the border color for a specific sheet, you can still do that by selecting the cells you want to format, clicking on the "Home" tab in the toolbar, and selecting the border style and color you want to use.

I hope that helps!