Dear Ross, I would say fluorescent blacklights work best.
I is suspecting the blacklight you got is the kind that look like
standard lightbulbs. Fluorescent ones, on the other hand, are
cylindrical,                           just like the bright white lamps
you sometimes see in workshops, ect.
I have never had any success in illuminating shells with a lightbulb
shaped
blacklight.

Kindest Regards,
James