October is the coolest month for Roger Barr. For a few happy weeks, grocery stores stock the object of his desire: Boo Berry.

That is the berry-flavored cereal that turns milk bluish, delighting generations of American kids—and some adults, too. The 35-year-old Mr. Barr eats a bowl of it nearly every morning, and for Halloween dresses up as the blue ghost with a porkpie hat seen on the front of the cereal box.

"It's the best cereal there is," says the Los Angeles-based website administrator. "I eat as much Boo Berry as I can."

Mr. Barr's problem is that he can't eat as much Boo Berry as he would like, unless he lays away a year's supply. Not long after Halloween, Boo Berry disappears from stores like an apparition.

The same sort of thing haunts lovers of Count Chocula and Franken Berry, the other two spectral cereals General Mills Inc. produces for the Halloween season.