You can use FileReader to read the Blob as an ArrayBuffer.

Here's a short example:

var arrayBuffer;
var fileReader = new FileReader();
fileReader.onload = function(event) {
    arrayBuffer = event.target.result;
};
fileReader.readAsArrayBuffer(blob);

Here's a longer example:

// ArrayBuffer -> Blob
var uint8Array  = new Uint8Array([1, 2, 3]);
var arrayBuffer = uint8Array.buffer;
var blob        = new Blob([arrayBuffer]);

// Blob -> ArrayBuffer
var uint8ArrayNew  = null;
var arrayBufferNew = null;
var fileReader     = new FileReader();
fileReader.onload  = function(event) {
    arrayBufferNew = event.target.result;
    uint8ArrayNew  = new Uint8Array(arrayBufferNew);

    // warn if read values are not the same as the original values
    // arrayEqual from: http://stackoverflow.com/questions/3115982/how-to-check-javascript-array-equals
    function arrayEqual(a, b) { return !(a<b || b<a); };
    if (arrayBufferNew.byteLength !== arrayBuffer.byteLength) // should be 3
        console.warn("ArrayBuffer byteLength does not match");
    if (arrayEqual(uint8ArrayNew, uint8Array) !== true) // should be [1,2,3]
        console.warn("Uint8Array does not match");
};
fileReader.readAsArrayBuffer(blob);
fileReader.result; // also accessible this way once the blob has been read

This was tested out in the console of Chrome 27—69, Firefox 20—60, and Safari 6—11.

Here's also a live demonstration which you can play with: https://jsfiddle.net/potatosalad/FbaM6/

Update 2018-06-23: Thanks to Klaus Klein for the tip about event.target.result versus this.result

Reference:

• https://developer.mozilla.org/en-US/docs/Web/API/FileReader#readAsArrayBuffer()
• https://www.w3.org/TR/FileAPI/#dfn-readAsArrayBuffer

I don't know why the author did wrap his Uint8Array in a new one... note that I don't really know either the deprecated BlobBuilder API, but one typo I can see in your code is that you need to wrap your TypedArray in a normal Array:

new Blob([new Uint8Array(buffer, byteOffset, length)]);

The Blob() constructor takes a blobParts sequence as first parameter, and then searches for BufferSource, USVStrings and Blob elements in this sequence. So when you pass a TypedArray, it will actually iterate over all the entries of this TypedArray and treat these as USVString (and thus convert their numerical value to UTF-8 strings in the Blob). That's rarely what you want, so better always pass an Array in this constructor.

Note that if you don't need to slice the buffer, it's probably better to directly pass it instead of using an intermediary Uint8Array (but still in a normal Array):

new Blob([buffer]);