Hi Ross,
As Lynn said, there is an article up and coming on this subject, so I won't
go into great detail here.  However, I'll bet your inexpensive blacklights
are screw-in type incandescent bulbs?  I have found these just about
worthless for this kind of work, though they are cheap (about $3 or $4 each).
 They produce too little ultraviolet radiation, and far too much visible
light.  What you need are fluorescent blacklight tubes (which fit into the
same fixtures as ordinary fluorescent light tubes).  18-inch, 15-watt tubes
can run $15 to $20 apiece, but they do the job.  The article provides more
details.
Regards,
Paul