This is done using the “Min File Size” setting on the Batch tab. This setting uses the image file size in bytes to determine which files are blank pages. For 200dpi, compressed, black & white TIFF images (the default format used by SimpleIndex) this is usually around 2000 bytes.
Depending on your scanner settings and how much black is in your images, blank pages could be significantly larger. If you are scanning small pages the margin for error will be less. You will want to do some trial and error testing to ensure the setting is right for your images.