this sounds like a simple fix. My concern is about lack of a straightforward troubleshooting options to a known issue such as this.
When you use Table/View as Source Type in Classic Report on a table containing a BLOB column, you get an error:
report error:
ORA-06502: PL/SQL: numeric or value error: character to number conversion error
You can change the Source type to SQL and change BLOB column to dbms_lob.getlength(BLOB_column) in the SQL Query or use the same in a View and use the View name in Table Name.
It sounds very simple and yet an unnecessary burden due to lack of documentation. If this is a known limitation of a classic report, wouldn't it help if APEX raised a straightforward exception that you cant use a table containing a BLOB column directly?