What is "dark conceit"?
By this I believe Spenser meant an allegory, in other words the Faerie Queen is meant to be read as an entertainment that also has a purpose. I found the following quote on enotes:
according to Spenser, the point of the "Dark conceit" and the book itself is “to fashion a gentleman or noble person in vertuous and gentle discipline.” The Faerie Queene thus functions as a conduct book used to train a perfect courtier. Each of the six books is devoted to the exploits of a knight who represents a particular virtue: holiness, temperance, chastity, friendship, justice, and courtesy.