Sadly the hidden Blade SSD is being seen by the OS installer trying to setup a Fusion Drive config. The best way is just to pull it out, I know a big pain! The other option is to create a bootable OS installer which you use to wipe the Blade SSD so it’s not recognized by the OS installer.
So… setup a bootable USB thumb drive by first formatting it to GUID and a Journaled file system (HFS+) copy the OS installer to it and then modify it following this guide [https://support.apple.com/en-us/101578|Create a bootable installer for macOS]
After booting up under it open Disk Utility and delete the partition on the Blade drive and I would format the drive MS DOS and now run the OS installer setting up the new SATA SSD drive