There is no easy way to do it. But this could work for you:
Within Finder create a Spotlight Search Folder with this search Query:
NOT (tag:a OR tag:b OR tag:c OR tag:d OR tag:e OR tag:f OR tag:g OR
tag:h OR tag:i OR tag:j OR tag:k OR tag:l OR tag:m OR tag:n OR tag:o
OR tag:p OR tag:q OR tag:r OR tag:s OR tag:t OR tag:u OR tag:v OR
tag:w OR tag:x OR tag:y OR tag:z OR tag:1 OR tag:2 OR tag:3 OR tag:4
OR tag:5 OR tag:6 OR tag:7 OR tag:8 OR tag:9 OR tag:0)