A nice way of rebooting a frozen Linux-based server when you do not have a power switch available is to use the SysRq-abilities. (Ubuntu Example.) Blackbox does not have a default macro installed for just that, but you can create a new macro and insert the following:
“<Alt-PRESS><Print Screen><Alt-RELEASE>RSEIUB”
This will create a working macro for writing any data in cache to disk, kill all processes (Sigkill), remount mounted disks and perform a reboot.