It’s blocking in the sense that it blocks one thread: the thread taken out of the pool of threads by the servlet container (Tomcat, Jetty, etc., not Spring) to handle your request. Fortunately, many threads are used concurrently to handle requests, otherwise the performance of any Java web application would be dramatic.
If you have, let’s say, 500 concurrent requests all taking 1 minute to complete, and the pool of threads has 300 threads, then 200 requests will be queued, waiting for one of the threads to become available.