Time Machine and the Blockchain! I use a cryptocurrency and it's blockchain is represented on disk by a single large lmdb database file.

The file is 72GB or so.

Each day the crypto node updates the chain with new transactions so the file is always getting added to.

I'm running Time Machine and seeing huge backups.

I thought Time Machine only backed up deltas of files? (i.e the difference between the old file and new file).

Does Time Machine instead backup a file completely, EVERY time it changes by a bit?

It seems that way.

Is time machine so that it will backup a full 70~ gigs a day each time my blockchain db changes slightly?

This seems wrong to me.

What do video editors do?? Aren't they always hitting this issue with large video files, 4K and whatnot?
Time Machine works on file level, so every time a file changes the full file will be backed up. As you‘ve found out it‘s not well suited for large files with frequent small changes. 

One way out would be to exclude the directory containing your blockchain file from Time Machine and use rsync to do backups.