Gardenia flowers are white, or shades of white, naturally. There are species and varieties that have a tinge of yellow or are a creamy light tan, but gardenia plants do not bloom any color other than white, or shades of white. As a gardenia flower ages and fades, it will turn to ivory, then brown, and then it dries and drops from the plant.
Although there is not a true blue-colored gardenia flower, a few varieties bloom so icy white that the blossoms can appear to have a very light blue/white color. The Kuchinashi, Prostrata and Radicans gardenia varieties bloom snow white and may show a faint light blue tinge at the flower interior when first opened.
Many fresh flower types, like carnations and daisies, can change colors by taking up colored water through their stems and into the petals -- but gardenias are not one of them. Because of the woody branch stem and thick, velvety petals, dying a fresh-cut gardenia blue has a low success rate.
Professional florists may be able to spray gardenias blue, using a commercial translucent spray dye that is formulated to be applied to fresh flowers. The fragrance will be missing, unless you spritz the fake flowers with gardenia perfume or essential oil, but artificial gardenia flowers can be found in blue colors.
Silk gardenia flowers may be available in blue, but you can also purchase white silk gardenias and use a fabric dye solution. Mix the blue dye with water. Make the color mixture lighter; then you can darken the blue color on your silk gardenias with multiple dippings. Soak the silk flowers in clear water first, shaking off excess water. This allows a more even coloring when you dip the gardenia in the dye. Experiment with the silk gardenias and the dye to achieve the color and look you want. Paper-crafted gardenia flowers can also be made or purchased in blue tones.