I'm allergic to the contrast dye, but could a CT scan still detect bladder cancer?

Maybe. Although ct scans are an excellent radiographic study, they can easily miss small bladder masses. If one should have a very large bladder mass, it will likely be seen. However, a cystoscopy is still required because the mass seen on ct scan does not neccessarily mean cancer. A tissue diagnosis with a biopsy is still required to make the definitive diagnosis.