The callback URL should be the one that redirects back to your site. So setup a callback url with blockchain like...
https://www.yoursite.com/callback/blockchain
Assuming you are using something like express in your app make a route like so.
app.get('/callback/blockchain', function (req, res) {
// Stuff here  
});
you will prob need to include
var https = require('https'); 
That way then you can set up your logic inside for example...
// Stuff here
 var options = {
    host : 'api.blockchain.info',
    path : '/some/path/',
    port : 443,
    method : 'GET'
  }
 var request = https.request(options, function(response){
    var body = ""
    response.on('data', function(data) {
      body += data;
    });
    response.on('end', function() {
      res.send(JSON.parse(body));
    });
  });
  request.on('error', function(e) {
    console.log('Problem with request: ' + e.message);
  });
  request.end();