GrapheneOS is 100% open source and supports reproducible builds. It is in no way a black box.
If you do not want anyone to hold your signing keys, you will need to stop using nearly all modern hardware and software.
This is not the fault of the Graphene team so much as a fundamental problem in the mobile chipset market.
Desktop/Laptop CPUs give you dramatically more control, especially with OpenSIL etc.
There are simply are no sovereign software solutions for modern mobile hardware.
The Linux distro I founded has zero blobs. Built completely from source and boots and runs on real laptops and desktops. That is what 100% open source looks like. Meanwhile every Android distro requires a vendor partition with dozens of black box blobs.