Chrome Extension to unblock a blocked url using webrequest

Question

I am trying to unblock a blocked URL using chrome.webrequest api,I went through the api documentation and implemented.It is blocking the url ,but after it ,when i run the unblock part of the code ,it is not unblocking the url.Below,is the code for the unblocking part.Is there a way ? chrome.webRequest.onBeforeRequest.addListener( function(details) { return { } }, {urls: ["url to be unblocked "]}, ["requestBody"]);