The blue row numbers in Microsoft Excel indicate that those rows are currently selected. This means that any action you take, such as formatting or deleting, will apply to the selected rows.

To deselect the blue rows, simply click on any other cell or row in the worksheet. The blue highlighting will disappear and the row numbers will return to black.

If you want to select multiple rows at once, you can click and drag your mouse over the row numbers to highlight them in blue. Alternatively, you can hold down the "Ctrl" key on your keyboard and click on individual row numbers to select them.

It's important to pay attention to which rows are selected before making any changes to your worksheet, as you don't want to accidentally apply formatting or delete data from the wrong rows.