"Blended so well" could mean that the frog had changed their outside color to match the color of the milkshake. Or, it could mean that the frog was blended into the milkshake, as in they were chopped up and combined well. This joke is playing on the word "blended" because it could mean two different things in this scenario.
The frog went unnoticed in the milkshake because it blended so well .