Wouldn't a 1980 Bianchi also have the Piaggio decal? Or what that right before the merger?

As to the decals, they can (and do) change color over the years. Many factors involved such as where a bike was stored, what sort of light it sat in mostly, weather, etc. So, what once may have been one color of blue, over years, may change to a completely different hue. I'm not contradicting anything, I'm just saying that decals can and do change color over time.

BTW, I've always had a thing about Celeste Green. You have a very beautiful bike!