Success! Applying the following change to context/Dockerfile:
14c14,16
- cert-sync /etc/ssl/certs/ca-certificates.crt
---
+ cert-sync /etc/ssl/certs/ca-certificates.crt && \
+ rm /usr/share/ca-certificates/mozilla/DST_Root_CA_X3.crt && \
+ update-ca-certificates
has backblaze connecting successfully in TrueNAS SCALE (and presumably elsewhere). That was the only change needed as well.
Edit: Note I did test a build of the Docker container without any modifications to verify the behavior before implementing the above fix.