Can a block of chocolate go bad? Can chocolate be untemperable? Is the chocolate cursed? Any suggestions would be most welcome. These experiences have shaken my confidence; I guess I was just enjoying beginner's luck in all my previous chocolate work.
Yes, a block of chocolate can most definitely "go bad," both in terms of flavour (any type of fat will eventually turn rancid; milk and white will spoil faster than dark) and in terms of temper. Like Kenneth T said, if your block got out of temper between Christmas and "recently," then it's useless for seeding. If you have a marble or granite slab, you could temper some by tabling it, and if you've been successful, use the tabled bit to precrystallize the rest of the choc block. If you have a cocoa butter block, you can try chef Eddy's method, my favourite for small quantities of chocolate:
Can chocolate be untemperable? If you got water into the melted chocolate and it seized, then yes, it is untemperable (can still use it in some baked stuff etc). Otherwise, whenever I had issues with crystallization, it was "operator error."
Is the chocolate cursed? Sometimes it feels that way, doesn't it. Chocolate sure does have a way of keeping us humble. Milk and white are more finicky usually. Sounds trite, but practice really does make perfect.
I am not familiar with the Chocovision machine, but my suggestion would be that you try tempering by hand. Seed with cocoa butter or chocolate that you are certain is in temper, or table the chocolate on marble to precrystallize without the use of seed. Always test the temper before pouring the chocolate in moulds. Hit your moulds briefly with a heat gun before pouring in the chocolate, so the chocolate is not shocked by a very cold mould. Make sure heat can escape quickly from the chocolate in the cavities, so that residual heat does not knock your chocolate out of temper.