I had the same issue, but then found that per the Guacamole Common Js spec, the OuputStream.sendBlob method expects a Base64 encoded string representing the blob.

Here's a little snippet from my quite early code to do this

const blobToBase64 = (blob) => {
    return new Promise((res, _) => {
      const reader = new FileReader();
      reader.onloadend = () => res(reader.result);
      reader.readAsDataURL(blob);
    });
  }

const sendBlobBasedOnMimeType = async (item, mimeType) => {
    const blob = await item.getType(mimeType);
    const blobAsDataUrl = await blobToBase64(blob);
    const blobAsB64 = blobAsDataUrl.split(",")[1];
    console.log("size of b64 blob", blobAsB64.length);
    const stream = guac.current.createClipboardStream(mimeType, "remote");
    stream.onack = () => {
      stream.sendEnd();
    }
    stream.sendBlob(blobAsB64);
  }