A common misunderstanding is Blitline's "asynchronous" nature. When you POST a job to Blitline, Blitline will return you data immediately, but that doesn't mean the job is "done", it simply means that Blitline has accepted your request and that the job will be done "soon" (usually counted in milliseconds). There are only 2 ways for you to know when a job is actually "done", postbacks and polling. 

With postback, Blitline will call YOUR server (if you have one) when the job is done. You give us a postback_url and Blitline calls that upon completion of a job. 

With polling, you effectively connect to Blitline and wait for the job to complete.