group_concat( column_json(dynamic_column )) return empty result

mysql> desc mysql.http_status_history;

+--------+-------------+------+-----+---------+-------+

| Field  | Type        | Null | Key | Default | Extra |

+--------+-------------+------+-----+---------+-------+

| id     | smallint(6) | NO   | PRI | 0       |       |

| status | blob        | YES  |     | NULL    |       |

+--------+-------------+------+-----+---------+-------+

select id,column_json(status)  from http_status_history;

 {"date":"2014-03-12 21:36:57","UPTIME":"10","ROWS_READ":"-437","MEMORY_USED":"1.18e6","SELECT_SCAN":"-1","OPENED_FILES":"76","HANDLER_WRITE":"-38","ROWS_TMP_READ":"-400","HANDLER_COMMIT":"-1","HANDLER_UPDATE":"-38","HANDLER_READ_KEY":"-438","INNODB_ROWS_READ":"53","HANDLER_TMP_WRITE":"-400","INNODB_LOG_WRITES":"1","KEY_READ_REQUESTS":"16","COM_REPLACE_SELECT":"-1","CREATED_TMP_TABLES":"-1","INNODB_DATA_FSYNCS":"7","INNODB_DATA_WRITES":"11","INNODB_DATA_WRITTEN":"230400","INNODB_DBLWR_WRITES":"2","ARIA_PAGECACHE_READS":"2","INNODB_OS_LOG_FSYNCS":"2","INNODB_PAGES_WRITTEN":"7","INNODB_ROWS_INSERTED":"1","HANDLER_READ_RND_NEXT":"-400","INNODB_OS_LOG_WRITTEN":"512","TABLE_LOCKS_IMMEDIATE":"14","OPENED_PLUGIN_LIBRARIES":"33","UPTIME_SINCE_FLUSH_STATUS":"10","INNODB_DBLWR_PAGES_WRITTEN":"7","ARIA_PAGECACHE_READ_REQUESTS":"113","ARIA_PAGECACHE_WRITE_REQUESTS":"6","INNODB_BUFFER_POOL_BYTES_DIRTY":"-81920","INNODB_BUFFER_POOL_PAGES_DIRTY":"-5","INNODB_BUFFER_POOL_PAGES_FLUSHED":"7","INNODB_BUFFER_POOL_READ_REQUESTS":"70","INNODB_BUFFER_POOL_WRITE_REQUESTS":"4"} |

| {"date":"2014-03-12 21:37:08","UPTIME":"11","BUSY_TIME":"5.9e-5","QUESTIONS":"1","ROWS_READ":"-435","BYTES_SENT":"33","MEMORY_USED":"1.18e6","SELECT_SCAN":"-1","OPENED_FILES":"76","HANDLER_WRITE":"-37","ROWS_TMP_READ":"-400","BYTES_RECEIVED":"57","HANDLER_COMMIT":"-1","HANDLER_UPDATE":"-37","HANDLER_READ_KEY":"-437","INNODB_ROWS_READ":"54","HANDLER_TMP_WRITE":"-400","INNODB_LOG_WRITES":"1","KEY_READ_REQUESTS":"16","COM_REPLACE_SELECT":"-1","CREATED_TMP_TABLES":"-1","INNODB_DATA_FSYNCS":"7","INNODB_DATA_WRITES":"11","INNODB_DATA_WRITTEN":"230400","INNODB_DBLWR_WRITES":"2","ARIA_PAGECACHE_READS":"2","INNODB_OS_LOG_FSYNCS":"2","INNODB_PAGES_WRITTEN":"7","INNODB_ROWS_INSERTED":"1","HANDLER_READ_RND_NEXT":"-399","INNODB_OS_LOG_WRITTEN":"512","TABLE_LOCKS_IMMEDIATE":"14","OPENED_PLUGIN_LIBRARIES":"33","UPTIME_SINCE_FLUSH_STATUS":"11","INNODB_DBLWR_PAGES_WRITTEN":"7","ARIA_PAGECACHE_READ_REQUESTS":"113","ARIA_PAGECACHE_WRITE_REQUESTS":"6","INNODB_BUFFER_POOL_BYTES_DIRTY":"-81920","INNODB_BUFFER_POOL_PAGES_DIRTY":"-5","INNODB_BUFFER_POOL_PAGES_FLUSHED":"7","INNODB_BUFFER_POOL_READ_REQUESTS":"71","INNODB_BUFFER_POOL_WRITE_REQUESTS":"4"} |

 select GROUP_CONCAT(cast(column_json(status) as char))  from  mysql.http_status_history;

+----------------------------------------------------------------------------+

| GROUP_CONCAT(cast(column_json(status) as char))                            |

+----------------------------------------------------------------------------+

| ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, |

+----------------------------------------------------------------------------+