|First things first. A "black light" will not work for most fluorescent minerals.
Blacklights are long-wave UV. They are usually un-filtered. They are also cheap.
What you need is short-wave UV. These lamps are filtered. They cost more.
Blacklights will make your laundry fluoresce, but they're not much good for most minerals.
Do yourself a favor and take the $20 you were going to spend on a "blacklight" and put it toward a good mineral light. (A 4-watt, by the way, is not a good mineral light. See below).
Decent short-wave lamps start at around $300.
Consider this the entry cost to the hobby. It's just like any other modern hobby. If you're going to fish much, you should invest in a decent rod and reel. If you're going to play golf, you want a good set of clubs. Collecting fluorescent minerals is no different, except I think it's more fun than golf or fishing. If you ask enough collectors, sooner or later you'll find one who will sell you a used short-wave lamp. The bulbs and filters do have a limited lifespan, though, so your best bet is to buy a brand new lamp.
Many people have asked me where they can get a short-wave ultraviolet light to view fluorescent minerals. I usually recommend to skip the tiny, battery-powered lamps and save your money for a good, powerful lamp-- you won't regret it in the long run. The company I work for used to sell the Ultralight 5000S, which was a very good lamp (powerful), but it seems the manufacturer is either not around anymore or has become unresponsive (looking for another manufacturer at the moment).
The Superbright 2000SW is still hard to beat for field collecting. The UltraLight is a bit large to haul around in the field, but if you hook it to a motorcycle battery and power inverter (carried in a backpack), it can be serviceable for field use, provided you're careful not to break the extra-large filter glass. It is a very bright lamp, that's for sure. I've taken it to at least one Franklin night dig (well, actually a couple) and it was good.
With either brand of lamp you should put a piece of chicken wire over the filter to add some protection against rocks. During field collecting it's all too easy to set the lamp face down on a rock and crack the filter.
Everyone thinks they'll never do it, and then it happens.
In the case of the Superbright, usually I see collectors put the wire mesh guard between the rubber feet and the lamp body, but I've also seen it put on outside the feet.
The biggest complaint I had about the old Superbright was the battery connector. The original one pulled out every 5 seconds or so during field use. It was maddening. I actually took mine, removed the original connector, and re-wired it with a screw-on military surplus connector. I had to drill a bigger hole in the case to accomodate it.
The revised Superbright II has a similar connector now, but I still like mine better because it's a garage job. My connector is also 100% metal, with no plastic that can crack into itty, bitty, useless fragments. UPDATE (30 May 2007): I recently spoke with Mr. Newsome via email; combined with my own, recent observations of the new Superbright, it looks like the the bugs have been worked out. The plastic connector on the new lamp is impact-resistant, made for heavy field use among the boulders. I understand that tests were done by dropping a 3-lb hammer onto the connector, and it continued to function.
For historical reference, the other major annoyance with the old Superbright was that the cigarette-lighter adapter used in the original battery pack often came loose. As in, every 10 seconds. It was either one end or the other that pulled loose, making a typical field-collecting trip little more than an intermittent string of cussing.
The light would wink out constantly, so I finally removed the cigarette-lighter adapter in my battery pack and just hard-wired it. No more problems occurred after that. So what if I can't use the AC adapter anymore? Mine burned out years ago anyway.
When I go in the field with a light that requires a power inverter, I duct-tape the battery clips to the battery so they won't pull loose in the backpack. That's important. Also, I don't think it would be good to have the terminals of a storage battery shorted inside your pack. No, make sure you tape those connectors in place.
As I said before, the newer Superbright is improved. It was a good product to begin with (for indoor use), but I really put mine through its paces in the field.