Is there a way to make a halogen Par Can into a blacklight with a filter/gel?
A normal non-led par can into a black light. Hopefully cheaper than buying a black light! 
Neither an LED or halogen lamp has any emittance in the UV (or very very little for the halogen)
http://www.hnuenergy.com/download/LED-VS-Halogen-visible.jpg
Look at the graph and the intensity as the lamp curves go into the violet color on the lower left. The intensity goes to zero as it gets to the ultraviolet portion on the left. These are typical for halogen lamps and for 'white' LED lamps.
There is no filter you can use to block visible light and get UV light from those lamps, because those lamps don't produce any UV light to begin with.
The only lamps that produces UV are a black light, or a UV LED. They make LEDs that produce UV light. I don't know if these are cheaper than black lamps, or not. You can do the research, I'm sure.
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