Could you point to how this is implemented and how it can be used? From the sound of it, you're trying to do something like rsync's running-window comparison?
You could consider building this storage system on top of BLAKE3's tree model. Namely you store blocks as small Merkle tree. And an image is basically a collection of blocks that has a different 'hat' on top of it. Unfortunately, BLAKE3 makes this hard, because the same block will end up having a different Merkle tree node depending on the offset at which it's stored.
Either modify the algorithm to your needs, and rename it.
Or just use something thats already suitable off-the-shelf. Plenty of merkle-trees out there.
Consider a crypto currency pow that did that without the chunk counter. It'd be trivially exploitably by precalculating all the tree but the chunk that changed per nonce.
Most OSes use 4K block sizes. To get the best dedup you have to hash every 4K block and lookup each one individually in a dedup table. Two VM images could both contain an identical 4GB file, but every 4K block of that file could be stored at different offsets in the VM images. A tree hash wouldn't let you dedup anything but identical sections stored at identical offsets, whereas a dedup table and 4K blocks allows you to dedup the entire file.