The blue topaz available are irradiated stones that can change color with application of heat. And this is probably what happened to your stone as well. A small price to pay for a lesson of a life time...
The current Jewelry Artist magazine has a really interesting blurb
on that. My understanding is that the radiation causes the atoms to
shift out of their lattice structure somewhat. This causes the
colour change. When it’s heated, the heat makes them vibrate more and
they tend to slip back into their original position in the structure.
This makes the colour change back to original, and is often
accompanied by a flash of light according to the article.
Paul Anderson