I feel sorry for you but loosing game probably happened to all of us and there is nothing one can do against. I assume that your shots with the 6,5x54MS were well placed in spite of the "dark on black" difficult conditions, however probably not immediately lethal, the reason for which the beast was able to go and die in a distant location; shifting to 450/400 is probably the best thing to do as "too much gun" is often the solution with strong game. I would personally go for 8x68S, 9,3x62 or 9,3x74R if I had to shoot bears from a high seat in low-light conditions; lighter calibers can do the job, of course, but I think in day light best, when one can 100% be sure of the shots' placement. I'm sure you'll get another black bear soon, at another occasion!
Louis