in disguise
Hiding the true appearance or nature of someone or something. A: "Well, if you want to spy on him, we'll have to go to the restaurant in disguise." B: "I've got some wigs from last Halloween—will that work?" Running late for work turned out to be a blessing in disguise. I would have been in the middle of that big multi-car pileup had I left the house on time.
in disguise
hidden behind a disguise; looking like something else. Santa Claus was really the little child's father in disguise. What I thought was terrible turned out to be a blessing in disguise!