Where is this CSAM stored? is it stored in nonsegwit (Layer 0 Bitcoin) or is it stored in witness (Layer 1 Segwit)? If it’s stored in Layer 0 Blockspace (nonwitness) then Blake-fork still would inherit the same data unless the fork is also temporal in height to a block state where this information is not yet written. If it exists in segwit, you simply don’t need to store segwit bits. You can just fork Segwit without breaking Layer 0 Bitcoin consensus and you can solve the issue without hardforking the chain via POW change.