using group_concat in PHPMYADMIN will show the result as [BLOB – 3B]

Looks as though GROUP_CONCAT expects that value to be a string. I just ran into the same problem. Solved it by converting the int column to a string like so:

SELECT rec_id,GROUP_CONCAT(CONVERT(user_id, CHAR(8)))
FROM t1
GROUP BY rec_id

Figured I’d share in case you were still having an issue with this.