Hey @jakevis, this is unfortunately the expected behaviour with the HTTP Request node and it has confused me too 
You’d need to return a JSON object in the headers field using an expression rather than write a JSON string. This earlier post of mine has an example of how such an expression could look like, in your case the expression should be something like {{ {"x-ms-blob-type": "BlockBlob"} }}:
This is the resulting header:
