Why does the architect of blob storage matter? The hash can be calculated as data streams in for the first write, before data gets dispersed into multiple physically stored blocks.

It is common to use multipart uploads for large objects, since this both increases throughput and decreases latency. Individual part uploads can happen in parallel and complete in any sequence. There's no architectural requirement that an entire object pass through a single system on either S3's side or on the client's side.