First, there is no officially described var. coerulea for Cattleya walkeriana, so someone applies that name descriptively if they subjectively think it has blue coloration. Blue ones do exist, or course, but that is not an officially described variety or form. Cattleya walkeriana var. coerulea could be informally applied to any bluer than average member of the species, including 'Blue Moon', but it isn't really correct to write the name that way. That doesn't stop everyone from doing it.
Cattleya walkeriana 'Blue Moon' (with that specific notation) indicates one particular plant that presumably has blue coloration, and all divisions or clones that come from it carry the same name.
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