This was something I didn't discover until this year, and I see a bunch of new haunters starting up, so I thought I'd post this as a public service.
There is really no such thing as an incandescent black light bulb for the purposes of a home haunter. A true black light is going to be the skinny fluorescent tube. They can be purchased in short portable sizes, but they will always be the fluorescent tube type (some are the compact spirals that can be used in regular sockets, but they are still the fluorescent kind). The best kind (used for theatrical and professional levels) are mercury vapor black lights, which use a UV emitting phosphor and an envelope of Wood's glass, but are very expensive.
The rounded bulbs that are sold in huge numbers around Halloween are not true black lights:
A black light may also be formed by simply using Wood's glass instead of clear glass as the envelope for a common incandescent bulb. This was the method used to create the very first black light sources. Though it remains a cheaper alternative to the fluorescent method, it is exceptionally inefficient at producing UV light (a mere few lumens per watt) owing to the black body nature of the incandescent light source. Incandescent UV bulbs, due to their inefficiency, may also become dangerously hot during use.
ALL OF THESE BLACK LIGHTS GET VERY HOT. They will be much hotter than normal light bulbs, and can be dangerous if too close to dry or flammable sources. They will melt plastic, and I actually
had an outdoor light fixture made of thick plastic warp and bubble due to one of those bulbs in it for 5 hours - which was amazing since the put out so little light.
"Soft" ultraviolet light in the UVA (where most black lights are) may cause wrinkles and skin cancer. Slightly "harder" UVB causes sunburn.[1] UV doesn't trigger our pupillar reflexes and might potentially cause opacification of the eye lens risking cataracts. This may occur after a life's worth of normal sunlight as well but might put night-club workers at risk.
http://en.wikipedia.org/wiki/Black_light for more info.