* Sirius is not a red dwarf: Black dwarfs are the theoretical final stage of red dwarf stars. Sirius, however, is a white dwarf, a much more massive star that has already gone through its red giant phase and shed its outer layers.
* White dwarfs cool over extremely long timescales: While white dwarfs do cool down over time, this process is incredibly slow. It's estimated that it could take trillions of years for a white dwarf to cool down enough to become a black dwarf.
* The universe isn't old enough: The universe is only about 13.8 billion years old. This means that there hasn't been enough time for even the oldest white dwarfs to have cooled down completely.
In short: While Sirius will eventually cool down and fade into a black dwarf, this will likely take longer than the current age of the universe. It's a process that will happen far in the future, long after our Sun has ceased to exist.
