Black boxing is when a complicated process is skipped over in reasoning. You supposed that mind uploading was possible for the sake of argument, to support a conclusion outside of the argument.
I see no reason, why uploading would be impossible. As I see no reason, why interstellar traveling would be impossible.
I have no idea how to actually do both, but that's another matter.
If the naturalistic view is valid, it is difficult to see a reason why those two would be impossible. But if the Universe is a magic place, then of course. It's possible that they are both impossible due to some spell of a witch, or something.
Still, I do assign a small probability to the possibility, that the consciousnesses is something not entirely computable and therefor not executable on some model of a Turing machine. But then again, the probability for this I see quite negligible.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.