What I would love to see in an SCM that properly supports large binary blobs is storing the contents using Prolly trees instead of a simple SHA hash.

Prolly trees are very similar to Merkle trees or the rsync algorithm, but they support mutation and version history retention with some nice properties. For example: you always obtain exactly the same tree (with the same root hash) irrespective of the order of incremental edit operations used to get to the same state.

In other words, two users could edit a subset of a 1 TB file, both could merge their edits, and both will then agree on the root hash without having to re-hash or even download the entire file!

Another major advantage on modern many-core CPUs is that Prolly trees can be constructed in parallel instead of having to be streamed sequentially on one thread.

Then the really big brained move is to store the entire SCM repo as a single Prolly tree for efficient incremental downloads, merges, or whatever. I.e.: a repo fork could share storage with the original not just up to the point-in-time of the fork, but all future changes too.