AFAIK there’s only one blob required to use the system. It’s the DDR4 training blob that is run once at boot as part of memory initialization. It’s not signed though so it could be reverse engineered and re-implemented if someone gets the motivation.
The other blob is optional and it’s for the HDMI output which you can enable by changing the DTB loaded at boot. That one is signed so re-writing would be a bit more difficult, but it can also just be ignored if the external HDMI is not needed.
Security is all going to be relative to your threat model. Do you want open-ness? Or maximum security. There’s no TPM chip or any secure-boot if that’s what you desire (attempting to prevent evil maid type attacks).
Not even the ARM Cortex A53 which does not have out-of-order execution like the newer A72 and such can still be vulnerable to spectre-like attacks (https://arxiv.org/pdf/2007.06865.pdf). The Raptor Blackbird you mentioned is also vulnerable to Spectre/Meltdown, POWER9 isn’t immune there. The Speculative Execution Impact For A 4-Core POWER9 Blackbird Desktop - Phoronix
In terms of upgradability, the CPU module is a removable on a DIMM-connector module. It currently is a quad core Cortex A53 as ruff mentioned above, with possibly upgrades being a dual core A72, a FPGA based module, and whatever comes up down the line.