blockdev: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/sbin/blockdev - from package: util-linux-ng

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/sbin/blockdev - from package: util-linux

You may install the required package with command apt-get install {package}

Error in other languages:
blockdev: bevel nie gevind nie
blockdev: comandă negăsită
blockdev: comando não encontrado
blockdev: commande introuvable
blockdev: command not found
blockdev: käsku ei ole
blockdev: không tìm thấy lệnh
blockdev: komanda nerasta
blockdev: Kommando nicht gefunden.
blockdev: kommandot finns inte
blockdev: komut yok
blockdev: nie znaleziono polecenia
blockdev: níor aimsíodh an t-ordú
blockdev: no se encontró la orden
blockdev: no s'ha trobat l'ordre
blockdev: opdracht niet gevonden
blockdev: parancs nem található
blockdev: perintah tidak ditemukan
blockdev: príkaz nenájdený
blockdev: příkaz nenalezen
blockdev: команда не найдена
blockdev: командата не е открита
blockdev: 命令找不到
blockdev: コマンドが見つかりません