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

For me, this helped (found it in this blog post):

In my case the parameter to GROUP_CONCAT was string but the function still resulted in a BLOB, but converting the result of the GROUP_CONCAT worked.

CONVERT(GROUP_CONCAT(user_id) USING 'utf8')

According to the MySQL documentation, CAST(expr AS type) is standard SQL and should thus be perferred. Also, you may omit the string length. Therefore, I’d suggest the following:

SELECT rec_id, GROUP_CONCAT(CAST(user_id AS CHAR))
FROM t1
GROUP BY rec_id

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.