"Black Lights" are actually ultraviolet lamps, usually portable handheld types, used by glass buyers in an effort to determine the age of clear glass. (They are also used to verify authentic vaseline glass but that will be discussed elsewhere.) It is believed by many that if clear glass glows yellow under a black light, it is proof that the glass article was manufactured prior to about 1915 when the formula from which glass was made changed. It would be great if it was as simple as that!
   And there is a thought that the property that causes the "glow" is the element manganese. There are other sources of flourescence--the phenomenon in which light is absorbed at one wavelength, then emitted at a lower energy wavelength along the ROY G BIV spectrum. Uranium salts used in vaseline glass will also cause the shift to green-yellow and much more efficiently. You can see the yellow-green glow from Vaseline glass in the daylight; whereas the "manganese" glow needs a darkened room. Uranium was also used to create some blues, such as the blue clambroth of Atterbury and the blue of the split horn of plenty vase that is blue on one side and clear on the other & in the formula for EAPG "apple green".    
    Actually, the results of the use of black lights is fraught with exceptions. In addition to the many, many mixed messages concerning the use of black lights on glass, different hues strike different eyes as different colors; yellow- green- blue- white and when different types of black lights are thrown into the mix, it's a veritable Ultraviolet Jungle out there!
    It would be great to have some hard and fast guide lines, but there simply aren't many easy rules! We once visited an elderly gentleman who had written a book on Gillinder glass but couldn't afford to publish it. He had lots of WESTWARD HO and other pieces of EAPG and he gave us a lesson in black lights. He had 2 of them; one was electric and one was a portable (like ours). He could differentiate between Westward Ho originals and repros with his 2 lights. But they all glowed what I call 'yellow' with our lamp. There are apparently lots of wave lengths and filters on UV black lights on the market and whatever the elements in glass that glow are (big controversy here also), they react differently to the various lamps.
    When looking at old glass with a black light, some see yellow as green & some see blue as white. For purposes here, we'll define "old" as pre-1916. To really use a light for benefit, a person needs to look at dozens of pieces of glass with their own black light and learn what 'known' old glass looks like then compare unknown aged pieces against that color of glow. After lots of looking, we believe the only 'TRUTH' is: 
            IF ITS NOT FLINT & IT DOESN'T GLOW YELLOW/GREEN
UNDER  A (OUR PORTABLE) BLACK LIGHT, ITS NOT OLD.
Ergo, if it's not flint & glows yellow, it may be old OR it may NOT be old. Yikes!
    To determine flint or lead glass, which generally glows "white" or "blue" or even "yellow" depending on the percentage of lead in the glass and the impurities mixed therewith, the characteristic "ring" when the glass is tapped is an additional consideration although, again, in some forms the relative weight of the item may more telling than the bell-tone. Flint pieces with a scoop shape, like bowls and some stems may give a bell-tone when thumped, but master salts and other "compact" pieces are never going to "resonate" although they may have a characteristic "ping" when tapped with a piece of metal. And then consider the variations in amount of flint (lead) in various pieces by various factories! Some patterns were made only in flint and really only a few patterns were made in both flint & non-flint & that info is generally available in the literature.
    When you get into old glass out of the general EAPG patterns, it can REALLY get hairy. Bottom line is that UV lights for telling age and flint/non-flint is pretty complicated! Best bet is to buy from an experienced, knowledgeable dealer if it really matters to you whether or not you're getting "flint" or original issues of EAPG. OR pay the price (tuition) to "learn by doing" like so many of us have.
See also Pattern glass ID.