So, building off of this, I too am curious as to why exactly a blacklight (that only produces heat) at night is not recommended for the turtles. If it put out light that'd be a different story, as I assume it may be enough to keep the turtles awake. From my understanding:
1. Turtles have a sleep cycle, just like you and I. You don't necessarily want to entice them to bask when they should be sleeping.
2. If the black light is only producing heat, it will not adequately heat the water in the tank. You would need a water heater for that. So providing the lamp for "heat" isn't necessary.