How can javascript upload a blob?

You can use the FormData API.

If you're using jquery.ajax, you need to set processData: false and contentType: false.

var fd = new FormData();
fd.append('fname', 'test.wav');
fd.append('data', soundBlob);
$.ajax({
    type: 'POST',
    url: '/upload.php',
    data: fd,
    processData: false,
    contentType: false
}).done(function(data) {
       console.log(data);
});

You actually don't have to use FormData to send a Blob to the server from JavaScript (and a File is also a Blob).

jQuery example:

var file = $('#fileInput').get(0).files.item(0); // instance of File
$.ajax({
  type: 'POST',
  url: 'upload.php',
  data: file,
  contentType: 'application/my-binary-type', // set accordingly
  processData: false
});

Vanilla JavaScript example:

var file = $('#fileInput').get(0).files.item(0); // instance of File
var xhr = new XMLHttpRequest();
xhr.open('POST', '/upload.php', true);
xhr.onload = function(e) { ... };
xhr.send(file);

Granted, if you are replacing a traditional HTML multipart form with an "AJAX" implementation (that is, your back-end consumes multipart form data), you want to use the FormData object as described in another answer.

Source: New Tricks in XMLHttpRequest2 | HTML5 Rocks

2019 Update

This updates the answers with the latest Fetch API and doesn't need jQuery.

Disclaimer: doesn't work on IE, Opera Mini and older browsers. See caniuse.

Basic Fetch

It could be as simple as:

  fetch(`https://example.com/upload.php`, {method:"POST", body:blobData})
                .then(response => console.log(response.text()))

Fetch with Error Handling

After adding error handling, it could look like:

fetch(`https://example.com/upload.php`, {method:"POST", body:blobData})
            .then(response => {
                if (response.ok) return response;
                else throw Error(`Server returned ${response.status}: ${response.statusText}`)
            })
            .then(response => console.log(response.text()))
            .catch(err => {
                alert(err);
            });

PHP Code

This is the server-side code in upload.php.

<?php    
    // gets entire POST body
    $data = file_get_contents('php://input');
    // write the data out to the file
    $fp = fopen("path/to/file", "wb");

    fwrite($fp, $data);
    fclose($fp);
?>