When you type "boot cdrom" at the 'ok' prompt, 'cdrom' refers to a device alias. Make sure that 'cdrom' is pointing to the correct device by typing 'devalias'. It's probably set right for you internal drive, but if you hook up an external one, you need to tell it explicitly where to boot from, either by specifying the device to the 'boot' command or creating or redefining a devalias and giving that as the argument to 'boot'.

On the other hand, are you sure that the Sun Blade 1000 is supported by Solaris 8?

Edit: Yup. Support was added in Solaris 8 10/00. That system is older than I remembered.