Is there a way I can get the border formatter node to work on these blank cells?
yes, you can create a “control table” using the “XLS Control Table from Cell Range”. There you can define tags for a certain cell range. Those tags are then used for the XLS Background formatter node.
Here is a small example workflow using the “XLS Control Table from Cell Range” node: Excel Formatting
I hope that helps. Let me know if you have more questions.