Sounds to me like the toilet valve never shut shore (or pump if turned on) water off after the fateful flush and filled the bowl to overflow. If your toilet seal is good how else would the black tank overfill and overflow......are you sure the black tank is actually full and that the toilet did not just overflow? I would also expect if the toilet flush valve is failing you would still see water filling the bowl unless maybe the flush valve had a plastic bit in the valve seal which has now cleared.....so no more valve problem.
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