For NodeJS (with node-fetch), you can get a buffer from blob using Buffer.from(await blob.arrayBuffer());.

However, you don't need to get a blob from node-fetch.
You can just get a buffer directly from the response.
And the info from blob.type can be obtained from the response header.
For example, to get a data url, using blob:

async function toDataURL_node(url) {
    let response = await fetch(url);
    let blob = await response.blob();
    let buffer = Buffer.from(await blob.arrayBuffer());
    return "data:" + blob.type + ';base64,' + buffer.toString('base64');
}

And without using blob:

async function toDataURL_node(url) {
    let response = await fetch(url);
    let contentType = response.headers.get("Content-Type");
    let buffer = await response.buffer();
    return "data:" + contentType + ';base64,' + buffer.toString('base64');
}

There is a pure JavaScript way that is not depended on any stacks:

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

For using this helper function you should set a callback, example:

blobToBase64(blobData).then(res => {
  // do what you wanna do
  console.log(res); // res is base64 now
});

I write this helper function for my problem on React Native project, I wanted to download an image and then store it as a cached image:

fetch(imageAddressAsStringValue)
  .then(res => res.blob())
  .then(blobToBase64)
  .then(finalResult => { 
    storeOnMyLocalDatabase(finalResult);
  });

I used this and it worked. below is server side code(NodeJS)

var contentType = 'image/jpeg';
    let base64String=req.body.img;
    let base64Image = base64String.split(';base64,').pop();
    let date=Date.now();
    fs.writeFile(`./uploads/${date}.jpeg`, base64Image, {encoding: 'base64'}, function(err) {
    console.log('File created');
    sourceFilePath= path.resolve(`./uploads/${date}.jpeg`);
   blobName=path.basename(sourceFilePath, path.extname(sourceFilePath));
   //console.log(sourceFilePath);
    blobService.createBlockBlobFromLocalFile(containerName, blobName, sourceFilePath, err => {
            if (err) {
                console.log(err);
            } 
            else {
                //resolve({ message: `Upload of '${blobName}' complete` });
                console.log("UPLOADED")
            }
        });