[mysqld_multi]
mysqld                  = /usr/bin/mysqld_safe
mysqladmin              = /usr/bin/mysqladmin
user                    = multi_admin
password                = multipass

[mysqld1]
server-id               = 1
port                    = 3306
user                    = mysql
pid-file                = /var/run/mysqld/mysqld.pid
socket                  = /var/run/mysqld/mysqld.sock
datadir                 = /var/lib/mysql
log-error               = /var/log/mysql/error.log
...

[mysqld2]
basedir = /usr
server-id               = 2
port                    = 3307
user                    = mysql
pid-file                = /var/run/mysqld/mysqld_blackhole_slave.pid
socket                  = /var/run/mysqld/mysqld_blackhole_slave.sock
datadir                 = /var/lib/mysql_blackhole_slave
log-error               = /var/log/mysql_blackhole_slave/error.log
...

sudo mkdir /var/run/mysqld
sudo chown mysql:mysql /var/run/mysqld