Canvas toBlob Shim, adapated with thanks from https://code.google.com/u/105701149099589407503/.

canvasToBlobShim.js

/**
 * Canvas toBlob shim, adapted with thanks from https://code.google.com/u/105701149099589407503/, 
 * from this chrome bug thread: https://code.google.com/p/chromium/issues/detail?id=67587
 */
(function(){
    /**
     * Convert a base64 image dataURL from a canvas element, to a blob.
     * @param {function} callback
     * @param {string} type
     * @param {number} quality
     * @param {base64=} dataURL The dataURL to use, rather than fetch from the canvas.
     */
    function dataURLToBlob(callback, type, quality, dataURL){
        dataURL = dataURL || this.toDataURL(type, quality);
        var bin = atob(dataURL.split(',')[1]),
            len = bin.length,
            len32 = len >> 2,
            a8 = new Uint8Array(len),
            a32 = new Uint32Array(a8.buffer, 0, len32),
            tailLength = len & 3;

        for(var i=0, j=0; i < len32; i++)
        {
            a32[i] = bin.charCodeAt(j++) |
                bin.charCodeAt(j++) << 8 |
                bin.charCodeAt(j++) << 16 |
                bin.charCodeAt(j++) << 24;
        }

        while(tailLength--)
        {
            a8[j] = bin.charCodeAt(j++);
        }

        callback(new Blob([a8], {'type': type || 'image/png'}));
    }

    if(HTMLCanvasElement && Object.defineProperty && !HTMLCanvasElement.prototype.toBlob)
    {
        Object.defineProperty(HTMLCanvasElement.prototype, 'toBlob',
        {
            value:dataURLToBlob
        });
    }
})();

Nice, thank you!