The frequent use of the term “blue Cattleya” notwithstanding, the flowers of variety coerulea Cattleyas are a color that is nowhere near the primary color blue the name suggests. The reason coerulea Cattleyas are referred to as blue is the bluer than is normal color of the flowers. The flower color of "blue" Cattleyas varies widely from cross to cross and may even vary considerably between orchids from the same clone. The color can be anything from an attractive shade of pink or pink-mauve to rich shades of purple-blue. AOS judges describe “blue” Cattleyas use a variety of descriptive colors such as lilac, rose blue, blue-lavender, lavender-blue, blue-violet, blue-purple or indigo blue. From these names, it should be apparent that most 'blue' cattleyas have a considerable amount of red in the flower color giving most 'blue' cattleyeas a color that is best descibed as a bluish shade of purple.
Blue flower color is rare in orchids, and very rare in Cattleyas growing in their natural range. While growing in the natural range, "blue" Cattleya are the result of extremely infrequent mutations producing a plant that is typically called "variety coerulea" Many Cattleya species have coerulea color forms, though some species such as the yellow flowered Cattleya dowiana either doesn’t have a coerulea form due to the yellow original color or the coerulea form is extremely rare.
CHEMESTRY AND GENETICS
In orchids, the range of colors possible for a flower is defined by a complex of anthocyanin and co-pigments within the cells of the flower while the actual color expressed is determined to a great extent by the floral pH. The blue color in many orchid flowers is known to be associated with an alkaline floral pH. Research on some blue orchid blue forms has sown that blue color increases as the floral pH becomes more alkaline. Experiments with the blue form of Phalaenopsis pulcherrima show that the pH resulting in the blue form of the flower is inherited in the ratios expected for as a single recessive gene. 
Blue Cattleya breeding expert Ervin Granier has conducted research which shows that the expression of coerulea coloration is very dependent on plant nutrition in at least the Cattleya alliance orchids he tested. His findings showed that the lack of a certain nutrient may suppress the expression of coerulea coloration significantly.. Fortunately, he says, most commonly used fertilizers such as Peters or Miracle Grow contain just enough of the nutrient to assure the full development of coerulea color. Adding more of the nutrient doesn't make flowers any bluer.
 Robert J. Griesbach, PhD, Breeding for Blue Flowers, Orchids, May 2005 (Volume 74, Number5)