Are you using the UDF functions to read the data from the blob column? Typically this is an symptom of something is wrong with the UDF installation and you should drop and reinstall them. We have seen this when UDFs are copied from one schema to another or restored from backup using a DB tool.
What version of Pega are you on? In later versions in the scripts directory of the media there is a generateudf script that can be used to install the UDFs. You would just drop the existing ones and then use the script to reinstall them.