We, like everyone else, take Grackles for granted. Just another black bird with a sharp eye. If the light is right though, their iridescence shines through. So many colors on one bird. It's mesmerizing. Take a look.
Birdwatchingdaily.com has a good article that explains how the colors appear iridescent.
Iridescent feathers are made up of complicated arrays of minute reflectors ... sometimes comprise several ordered layers of melanin granules, air cavities, or both. The structures are sometimes twisted, so they work like the crystals of an infinitesimally small chandelier, absorbing and reflecting a range of wavelengths.
In male peacock feathers, reflectors are spaced and shaped to reflect different wavelengths, producing an array of hues. Similar processes create iridescence in grackles, hummingbirds, and other shiny-feathered birds. The effect enhances some wavelengths and cancels out others. Iridescent birds are shiniest on bright days when the sunlight’s intensity is high ...The noniridescent colors arising from feather structures are white, blue, and occasionally green.
The article also tells us how the color blue occurs in birds.
Unlike virtually every other feather color, no pigment turns feathers blue ... In the cloudy zone of blue feathers, the melanin and air cavities are so close that the distance between them is shorter than a wavelength of light.
For a complete explanation and the full article, see here.