You can specify an arbitrary commit hash with the ref
parameter, but you cannot specify a blob hash instead of a file path. The reason is that filters and other attributes can be applied to files in the repository, and the path is needed to determine the appropriate content. The same blob located at different paths could be filtered differently, so a blob isn't sufficient to determine the correct contents.
If you need to fetch the raw contents of the blob without filtering or other attributes being applied, then you'll need to clone the repository.