Because the post editor uses postbacks, this doesn't feel like a native app.. but it's the best I can do w/ the design.  This will add a key mapping for ctrl-s so that it does the same as the "Save and Continue Writing" button.

Add this to the bottom of ~/ControlPanel/Blogs/CreateEditBlogPost.aspx

<script language="javascript">

var isSaving = false;
function DoKeyCommand(evt)
{
  var key = (evt.which || evt.charCode || evt.keyCode);
  var stringKey = String.fromCharCode(key).toLowerCase();
  var cmd = '';
	
  if (!evt || !evt.ctrlKey) return true;
  
  switch (stringKey)
  {
    case 's':
    {
      if (isSaving) return false;
      isSaving = true;	  
      if (evt.preventDefault) {
        evt.preventDefault();
        evt.stopPropagation();
      } else {
        evt.returnValue = false;
        evt.cancelBubble = true;
      }
      WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions("ctl00$ctl00$TaskRegion$Editor1$SaveEditButton", "", true, "", "", false, true));
	
      return false;
    }
  };
  return true;
}
if (document.addEventListener)
{
  document.addEventListener("keypress", DoKeyCommand, true);
}
else if (document.attachEvent)
{
  document.attachEvent("onkeydown", function(evt) {DoKeyCommand(evt)});
}

</script>