You get this error when you try to make a request from your site to a site on a different domain (or sub-domain). It is possible to configure the other site to allow you to make requests (see Mozilla documentation), although this feature is not supported everywhere.
An alternative to normal AJAX calls is JSONP, which requires the server to support it but doesn't have cross-domain restrictions. It turns out that Blogger does support this, so you just need to modify the URL to
var url = 'http://isimpledesign.blogspot.com/feeds/posts/default?alt=json&callback=?'
and jQuery will handle it properly. (Fiddle)
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