04 October 2018
You may use the IPictureObjectsInfo.BlacknessPercentage property.
Using the following code in the Rule script you'll be able to know the percentage of black pixels inside your field:
int blacknessPercentage = Context.Field("<your field name>").Regions.Picture.AnalyzePageObjects(null).BlacknessPercentage;
Then, based on the some threshold, you may determine if the field is blank or not.
In general, if you use your own customized export, you can include or exclude from it any fields, but if you are using the built-in FC export, to suggest to you better ways of exporting/non exporting of such fields, we'll need to look at your project and some sample images.