PHP is synchronous language by design so it must wait for each line to be executed. If you call post request with 15s timeout you must wait untill webservice send response or 15s timeout limit.
As I see in the core blocking parameter will not spawn process but simply do not parse response saving you a little time.
https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-requests.php#L630
You can make request with very small timeout as @mmm said or spawn process using exec function.
exec('curl --data "param1=value1¶m2=value2" http://url.com > /dev/null 2>&1 &');
If you do not want to wait for response but you will need it eventually you can use promise design pattern. https://github.com/reactphp/promise