I'd like to see the code, then perhaps I can spot what is happening. There is typically not a problem doing this; however there is a recommendation that the field length of your BLOB (graphic) field should always be 1, even though Paradox allows for greater sizes. I seem to remember that it was believed to cause problems, but I can't remember what type or under what circumstances. It might be worth a try to check that and change it if necessary.
If you are dealing with "invalid blob header," I would assume that the MB file is corrupt. That the situation you are dealing with is abnormal.
(Incidentally, the ChimneySweep product referred to in my signature is a Paradox oriented product, so "I know whereof I speak.")
Unfortunately, I'm zeroing in on the word, header. If the header of a blob-file is damaged, then it's very likely that the file has been overwritten, or scribbled, or "ScanDisk found errors on this drive and fixed them all" ... and the data might be (ummmm, err, that is to say...) reallygone.