Anything that goes through an HTTP(S) interface must be non-blocking. The HTTP protocol is not a continuous-connection protocol, but rather one that connects, submits data, gets a single response returned as quickly as possible, then closes the connection until it makes the next request (if any). That's a basic characteristic of the Internet and not a
Java or Spring - or cloud - limitation.
Some might argue that anything that can block shouldn't be called "micro", for that matter, but essentially if you want "blocking", you'll have to fake it the same way that AJAX fakes a "continuous" updating connection for HTTP.