If you're talking about old-school fluorescent-tube black lights, the bulb type designation will have a -BLB suffix, like F15T8-BLB for a typical 15-watt tube,

If you're talking about old-school fluorescent-tube black lights, the bulb type designation will have a -BLB suffix, like F15T8-BLB for a typical 15-watt tube, as well as a visibly dark phosphor. A germicidal tube would be G15T8, and have a clear quartz envelope with no phosphor coating. All bets are off if you're using LEDs.