Black Light can be used for spectacular photography or just for having some photographic fun, but if you just want to try out a quick trick for testing your home for bacteria there is a way to do it for a couple of cents.
Turns out that a certain mix of sharpie ink will block all light but back light. The folks at Hefty.co made a quick tutorial on how it’s made.
You would need a blue Sharpie and a purple Sharpie and some tape. Applying two blue tape layers and one purple tape layer will act as a filter for the smartphone flash. In total darkness shining that flash onto anything will reflect any black light (or fluorescent emittance) from found objects.