First, I need to remind you that if the BMC update is done incorrectly (i.e. if the instructions are not followed exactly, including decompressing the image-bmc file BEFORE transferring it to the BMC, or if your power fails in the middle of the update process), you will need to externally flash the ROM. Given the realities of COVID-19 worldwide, I wouldn't update the firmware at this point unless you knew you had all the parts on hand to be able to recover a bad Flash (Beaglebone Black / Raspberry Pi, SOIC-16 clip, wires, and a power supply).
That being said:
You need to scp the image-bmc file to /run/initramfs on the BMC, like so:
scp image-bmc root@<blackbird BMC IP address>:/run/initramfs/
Make sure the image-bmc file is ~32MB in size before you do this, and that the host power is off.
That will put the file in the right spot on the BMC. Once that is done, SSH to the BMC and:
fw_printenv
<write down the MAC>
reboot
WAIT 15 MINUTES BEFORE DOING ANYTHING, INCLUDING POWERING UP OR TOUCHING THE POWER CORD
After this delay, SSH back in to the BMC and run the fw_setenv commands as mentioned in the instructions.
If you did it correctly, the BMC will now be online, but the host will not IPL until you update the PNOR. There is nearly zero bricking potential there however, so it should be simpler for you to follow (in fact, I would recommend going to https://<BMC IP address> and using the new point+click firmware updater -- it's a lot easier!)