The mysqldump command with both the --xml and --hex-blob options will output blob data encoded as hexBinary.  
The proper XML datatype is xs:hexBinary.  
The correct XML datatype is specified be setting the xsi_type attribute equal to xs:hexBinary for each encoded element.


client/mysqldump.c:
  Bug#19745: mysqldump --xml produces invalid xml
  - Moved hex-blob code to it's own function.
  - Rewrote print_xml_tag function to accept zero or more attribute/value
    pair(s) which are appended to a separate tag name.
  - --xml option respects the --hex-blob option by ouputting blob data in hex
    encoded format.  Each hex encoded field tag will also contain an xsi:type
    attribute with an xs:hexBinary value.
  - --extended-insert and --xml are mutually exclusive. Otherwise, the xml file
    will contain INSERT commands.
  - Minor comment cleanup.
mysql-test/r/mysqldump.result:
  Bug#19745: mysqldump --xml produces invalid xml
  -Added results.
mysql-test/t/mysqldump.test:
  Bug#19745: mysqldump --xml produces invalid xml
  -Added test.