In a master-detail screen when I copy a master row, a detail containing blob data is not copied. The blob field is set up with a default lookup as ExcludeFromAllFields=True and OnCopyAction=Copy. In the data structure the field is added manually and has IsWriteOnly=True. Is this last parameter why it is not copied?
Yes. IsWriteOnly = True means data are never loaded. The data structure field is only used when data are uploaded so they can be saved to the database. You have to load the data e.g. by setting up BlobLookup which downloads the data using a lookup instead of taking it from the loaded data.
That all means that the such data are not loaded, thus not being copied.