UV bulb question

there are two types of UV tubes: Black light & Black Light-Blue glass

GEs website says the difference btwn BL and BLB is that BLB have dark
glass which traps visible light producing the maximum amount of UV?
this doesnt make much sense to me...having an opaque glass will just
trap visible light not generate more UV, right? or not? thx




http://www.gelighting.com/na/home_li...q_ballasts.htm

Blacklight-blue bulbs are designed with a special deep-blue glass that
filters out visible light, producing the maximum amount of ultraviolet
light possible. If you're looking for a blacklight that produces the
minimum amount of visible light for an especially sensitive area,
blacklight-blue bulbs are an ideal choice