Blue fireflies? What?

When I was a kid visiting my friend's house, at evening, LOTS of fireflies appeared. And I do mean LOTS. Anyway, a single one stood out between the others... it was glowing blue. Does anyone have any idea what was up with that? I can say for sure its glow was blue because two other people saw it as well, and when I caught it everyone marveled at its colour (including three adults, mind).