Since the GPU starts before the CPU, you need a blob to boot the GPU before you can start open code on the CPU. There is open GPU boot firmware being worked on but it isn't ready yet and relies on GCC support for the GPU, which isn't mainlined yet and the authors don't intend to mainline. So probably RPi will need blobs to boot for the foreseeable future.

https://github.com/librerpi/rpi-open-firmware https://github.com/itszor/vc4-toolchain/issues/7