Sorry to hear about your mother's diagnosis. First, very few urologists specialize ONLY in bladder cancer treatment. But you DO want one who treats a lot of bladder cancer. If you do not feel comfortable with her current urologist, of course, now would be the time for a change. If you do feel comfortable you might want to wait until you have a firm diagnosis before seeking a second opinion since you would know then exactly what questions you might have.
As for Johns Hopkins, I am not personally familiar with the urologists there. However, these would seem very qualified to treat bladder cancer:
urology.jhu.edu/bladder/member.php?trinitybivalacqua/
urology.jhu.edu/bladder/member.php?michaeljohnson/
I note that Dr. Armine is based at Sibley in DC....I believe that the others listed are in Baltimore.
Your mother is fortunate to have you at her side. Best of luck to you both!
Sara Anne