(Graph Presented) We show that trivalent manganese, Mn3+, imparts an intense blue color to oxides when it is introduced at dilution in trigonal bipyramidal coordination. Our optical measurements and first-principles density functional theory calculations indicate that the blue color results from an intense absorption in the red/green region. This absorption is due in turn to a symmetry-allowed optical transition between the valence-band maximum, composed of Mn 3dx2-y2,xy states strongly hybridized with O 2p x,y states, and the narrow Mn 3dz2-based conduction-band minimum. We begin by demonstrating and explaining the effect using a well-defined prototype system: the hexagonal YMnO3-YInO3 solid solution. We then show that the behavior is a general feature of diluted Mn3+ in this coordination environment.