The Blue Fish from One Fish Two Fish Red Fish Blue Fish.
She is seen on the cover of the book with the one fish, two fishes and the Red Fish and the first page with the same fish from the cover.
There seems to be another blue fish as well on the next scene and another blue fish that was in on the big red father fish spanking a fish (which is the yellow fish). (The third blue fish and the yellow one are possibly the red father fish's two children/two sons.) The narrator Kay about it says, "And some are very, very, bad!". Kay continues and asks why the fish are "sad, glad, and bad". Her answer is that she does not know then tells the reader to "go ask their dad" and see if their dad knows why. (In summary, she --Kay-- says, "Why are they sad, glad, and bad? I do not know. Go ask your dad".) This occurs on page 7 of the book.
Before that, she (along with the Red Fish) appear on pages 4 and 6. On page 6 --Beginner Book Video only-- she --along with the Red, the Old, and the New Fish-- appear when Kay (as the narrator) recaps the previous fishes. That is, when she says, "Yes. Some are red and some are blue. Some are old, but some are new".