You need to read all the rows in a loop, and concatenate them onto the variable.
$query = "SELECT * FROM blogs";
$query_params = array();
$tags = '';
try{
$stmt = $db->prepare($query);
$result = $stmt->execute($query_params);
while ($row = $stmt->fetch()) {
$tags .= $row['tags'];
}
}
catch(PDOException $ex)
{
die();
}
echo $tags;