Despite the promising expectations of poly(fluorene) (PF)-type materials as efficient blue-light-emitting polymers, the devices based on these materials are not yet fully utilized. Under prolonged operation of the devices, the PF-type materials undergo degradation with the appearance of a long-range emission around 2.2–2.3 eV. As a consequence, the emissive color changes from blue to green with a decrease in the device efficiency. Here, an innovative approach that leads to a new blue-emitting polymer with remarkable color stability is reported. By modifying the chemical structure of PF to inhibit the formation of keto defects, it is demonstrated that the devices exhibit excellent color stability. This new blue-emitting polymer, poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta-[def]phenanthrene)) (PCPP), emits a stabilized, efficient blue electroluminescence without exhibiting any peak in the long-wavelength region even after prolonged operation of the devices in air.