I've tried looking for something exactly like this before to no avail. Really awesome assuming it can deliver!
That said, being more transparent in the Readme on Github about the browsers that are currently supported and that plan to be supported would be appreciated. Blob is only Chrome 21+, Firefox 13+ and IE 10+ according to MDN (https://developer.mozilla.org/en-US/docs/DOM/Blob), and judging by the "FileBlob API is not supported" comment under the TODO section, there isn't any backwards compatibility yet. How far back are you planning on supporting?