"The slice method previously had different semantics, which differed from both Array.prototype.slice and String.prototype.slice [ECMA-262]. This difference was an oversight which has been corrected in this edition of the specification. Some user agents implemented the previous semantics, notably Firefox 4, Chrome 10, and Opera 11. These user agents have agreed to vendor-prefix their slice implementations in subsequent releases. A Blob slice operation occurs synchronously."
Please remove this note.
Marking this fixed. http://dev.w3.org/2006/webapi/FileAPI/#dfn-Blob