Using the same semantics as toDataURL (image/png is the default output), add Blob getBlob([Optional] in DOMString contentType, [Optional] args...).
Currently, an image must be copied to a string using toDataURL, then to an array buffer using base64 decoding, before it can be run through createObjectURL. getBlob would reduce the number of operations.
Support for Canvas toBlob is likely to come from Chromium:
This API has been renamed to toBlob: https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob