Good question! I've noticed that Gibson has been turning out less solid black LPs over that past 15 years and have been wondering why. I worked in aircraft, marine and auto coatings (painting) for nearly 5 years and your question peaked my interest. (My dream guitar is a tux Black Beauty.
There's a lot involved in answering this actually, and things have gotten more complicated as everyone is switching to polymer coatings - it has introduced a wider color selection of pigmentation options for guitarists.
Fender, PRS, Gibson - all have to rotate the color selections for recurring production models and the dark/solid-black is the more problematic finish. Reason being, they avoid spraying white and black (lighter and darker shades) on the same day. It's doable with separate booths, but not efficient. It's just easier to do all similar shades and color combos about the same time.
Fender has simplified the problem by doing the classic strat and tele colors in California and the more modern-contemporary colors in Mexico, thus splitting up the finishing schedules. Gibson decided to split its manufacturing between Memphis and Nashville and do the solid bodies in Nashville and semi/hollows (and CS mods) in Memphis. When the production cycle calls for black, they send the rough finishes to Memphis with ES models that come in solid black. They fit LPs into the finishing schedule with ES-335s, 339s, LPs, limited editions/customs for black at the same time.
Solid black, while popular, doesn't sell as big as other colors on some guitars. It's also more popular for some models more than others. It does appear that the popular demand for black LPs has dropped over the past 15 years. Used to be Gibson turned out a lot of black LPs every year - Black Beauties, LP Pros, and customs.
It looks like they dropped black from the LP production line, but that doesn't mean they dropped black completely. We just have to keep our eyes peeled for their limited runs and customs. I'm sure the black LP will never go away, they're just saying they've discontinued it from their main LP production line.