/var/lib/dpkg/info/blackbox.postrm has its generated debhelper block
only under "remove" case

    case "$1" in

      remove)
    # Automatically added by dh_installmenu
    inst=/etc/menu-methods/blackbox
    if [ "$1" = "remove" ] && [ -f "$inst" ]; then chmod a-x $inst ; fi
    if [ -x "`which update-menus 2>/dev/null`" ]; then update-menus ; fi
    # End automatically added section

      ;;

I think debhelper normally expects its code to run always (and checks
itself what dpkg action).  In this case I think the update-menus is
supposed to run always (though I think also it goes from a trigger so no
actual ill effect from its restriction).