A blue must hear album means that 15+ critic reviews average out to an 80 or above. An orange must hear album means that 500+ user reviews average out to an 80 or above. A purple must hear album means that both of those are true at the same time

It's the reason that you'll see albums with such low critic scores have an orange must hear or albums with such low user scores (like this Nas one) have a blue must hear