When using Illustrator's blends to generate an animation between frames containing transparency, after making the blend you should open the Transparency palette and turn off the Knockout Group attribute. Blends are by default created as Knockout Groups because it gives the best results for blends that are meant to look like a single object fading from solid to transparent. But when the frames are meant to be viewed as separate objects, it cancels out the internal transparency.

In this animation, transparency is used to make the stars and "Good night" text gradually fade in as the sky darkens. This is done by including the text and the symbol instance that contains the stars in every frame, but making them transparent in the daytime frames. Below is how the blend appears with Knockout Group on and off.

The four key frames

Knockout Group On

Knockout Group Off