Actually it's even easier than that. Files are already Blobs (anything that has getBlob() can be passed in to any function that expects Blobs). So the code looks like this:
var folder = DocsList.getFolder('path/to/folder');
folder.createFile(Utilities.zip(folder.getFiles(), 'newFiles.zip'));
Additionally, it won't work if you have multiple files with the same name in the Folder... Google Drive folders support that, but Zip files do not.
To make this work with multiple files that have the same name:
var folder = DocsList.getFolder('path/to/folder');
var names = {};
folder.createFile(Utilities.zip(folder.getFiles().map(function(f){
var n = f.getName();
while (names[n]) { n = '_' + n }
names[n] = true;
return f.getBlob().setName(n);
}), 'newFiles.zip'));
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