So the problem is that you want to upload a base 64 image and you have a blob url. Now the answer that will work on all html 5 browsers is: Do:

  var fileInput = document.getElementById('myFileInputTag');
  var preview = document.getElementById('myImgTag');

  fileInput.addEventListener('change', function (e) {
            var url = URL.createObjectURL(e.target.files[0]);
            preview.setAttribute('src', url);
        });
function Upload()
{
     var myCanvas = document.getElementById('MyCanvas');
     var ctx = myCanvas.getContext('2d');
     ctx.drawImage(preview, 0,0);
     var base64Str = myCanvas.toDataUrl();
     $.ajax({
         url: '/PathToServer',
         method: 'POST',
         data: {
             imageString: base64Str
         },
     success: function(data) { if(data && data.Success) {}},
     error: function(a,b,c){alert(c);}
     });
 }

So the problem is that you want to upload a base 64 image and you have a blob url. Now the answer that will work on all html 5 browsers is: Do:

  var fileInput = document.getElementById('myFileInputTag');
  var preview = document.getElementById('myImgTag');

  fileInput.addEventListener('change', function (e) {
      var url = URL.createObjectURL(e.target.files[0]);
      preview.setAttribute('src', url);
  });
function Upload()
{
     // preview can be image object or image element
     var myCanvas = document.getElementById('MyCanvas');
     var ctx = myCanvas.getContext('2d');
     ctx.drawImage(preview, 0,0);
     var base64Str = myCanvas.toDataURL();
     $.ajax({
         url: '/PathToServer',
         method: 'POST',
         data: {
             imageString: base64Str
         },
     success: function(data) { if(data && data.Success) {}},
     error: function(a,b,c){alert(c);}
     });
 }