Can search results be returned in JSON format, rather than in HTML?

Yes, use the debug mode with query parameter debug=true in your HTTP request to see your results in JSON format.

Example:-

https://localhost:8443/search?query=web&debug=true&public=true

Using the search API, the results will be in JSON by default

https://localhost:8443/rest/v2/api/search?pretty=true&query=*&public=true

To learn more on search in SearchBlox read: Searching in SearchBlox