Black glass is a term used for very dark colored glass as well as glass that is actually black. Usually a very dark purple or a darkly colored bottle of any color may seem to be black. The true color can be determined by holding the item to the light: usually a thinner section will provide the color of the glass. The exception to this is modern glass, such as that manufactured for the bathroom, where black vanity surfaces are black through and through.
In old bottles, the dark color provided some protection of the contents from deterioration by light. Many items of food and drink are sensitive to the effects of light over time. It made sense to protect the contents from light damage after spending the time to prepare the foods and bottle them. I am sure the effects of light were known to those who worked in the food preservation business over the centuries, even if that knowledge is no longer in circulation now due to our modern food systems.
Some wine bottles have always been dark in order to protect the contents. Here, the question is why some wines can be bottled in light-colored bottles. But vintners apparently know which wines require this kind of protection, and which do not.
In some cases, the practicality of using black glass jars and bottles is probably overridden by the necessity of showing the product to the purchaser. Jams, jellies, potted cheese and meat: how many of these would be in black glass instead of clear glass if the consumer did not require seeing the contents before buying it.
Beginning in the twentieth century, black glass was used for decorative purposes. With Art Deco, stylish mantle ensembles in black glass were made: clock and two candlesticks in a matching style, or a black glass figure and two glass candle holders. Smoking ensembles in black glass were also available: cigarette and/or cigar boxes, one or more ashtrays, and a lighter. A larger set might include a humidor especially for cigars, match holders for unused matches and match safes for used matches, and even cigarette holders. Hopefully the black glass would hide the residue left from the smoking materials, although that hope may be in vain.
It is not beyond imagination that one looking for black glass would also find lamp bases as well as vases, a decanter and glasses for entertaining, animal figures like black panthers, which were very popular at the turn of the 20th century, etc. With the iridized finish used in carnival glass, a black glass base item would become very beautiful, and perhaps more acceptable than the same item in plain black. In Victorian times, black meant mourning and was seldom used for items in everyday use. When the Victorian strictures were thrown off, one of the effects was to make black a chic new color and it was incorporated in the new, stream-lined interiors in the era that followed.
Black glass is sometimes used as the core of a glass paperweight, and the clear glass around it embellished with specks of color or air bubbles to make a design. The black background makes a dramatic backdrop for the rest of the glass added to the paperweight.
Modern black glass appears in kitchen, dining room and bathroom. Consider dining on a black glass table instead of clear glass. The table settings and serving dishes will sparkle more for being on a dark surface, and the feeling of falling through the table to carpet is ameliorated for those with vertigo. Black glass vanity tops are available for the bathroom. Black glass in the kitchen comes as the front panels of ovens, dish washers and refrigerators. It gives a modern look to the kitchen, and hides the perhaps less-than-immaculate oven from the view of nonresidents. At least that is the reason I would have a black front on my oven. Otherwise, the black glass shows fingerprints and water spots, and requires a non-abrasive cleaner, which is the down-side. It is amazing how much dirt you can hide on white appliances that really stands out on black surfaces. Keep that in mind if you go with black glass in your kitchen.