Since nobody has answered correctly, I'll give the answer: it was Henrietta singing it to Lady Aberlin.
To explain further, it was in Episode 1088, from the 1970 season. Betty Aberlin visits Fred and gives him a black angel fish for his aquarium (she is also wearing black, but I don't know if there is any significance to that). In the NOM, Lady Aberlin dresses as a black angel fish, who swims around the world searching for owls and pussycats. X and Henrietta are afraid that the fish is going to eat them. But then the fish says that she doesn't eat owls and pussycats, but she kisses them. When she kisses X and Henrietta, they realize that it's Lady Aberlin. Henrietta then sings the "You are pretty song" to Lady Aberlin, but of course sings it with meows, "Meow meow pretty, meow meow black . . . " Lady Aberlin then points it out that she can sing the same song to her, and then sings the familiar version. X the Owl then points out that nobody has ever sang a song about how he is blue, so Lady Aberlin then sings a version of the song for X. Later in the episode, back at Fred's house, Fred sings that song to the black angel fish that Betty gave him. So, this episode uses 4 different versions of that song, and is first sung by Henrietta!