Try typing $.ajax("https://blockchain.info/address/15cNko3ZtmYCba8GoaYsZ6GWFy1VCLgFji?format=json");
in your browser console in your application development tab.
You're likely to receive this error as response :
XMLHttpRequest cannot load https://blockchain.info/address/15cNko3ZtmYCba8GoaYsZ6GWFy1VCLgFji?format=json. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access.
This is a CORS related issue which is a whole topic on itself so I suggest you google this and understand its implications.
Next, if you can control CORS settings on the domain where you're trying to fetch json from, then you need to allow cross origin requests from your web application domain, this is possible when using an amazon S3 bucket, another web application you designed, etc...
If you can't, then I'm afraid you'll have to use a Meteor.method client side to reach your Meteor server where you'll fetch the json with HTTP.get then send it back to the Meteor client.