Wikipedia has a tiny stub on Biledulgerid that—on the basis of its 19th century source—treats it as an actual kingdom in early modern northwest Africa. This early 18th-century source, meanwhile, treats it as a vast empire comprising 12 regions from Mauritania to the edge of Egypt.
My assumption is that it's just early modern European geographers playing a game of telephone from an account by one explorer one time who stopped by the area for some dates. (The name for this Empire of the Central Sahara is apparently a mangling of the Arabic for "Land of Dates", apparently coined by the same marketing geniuses who came up with "Greenland".)
Any of you guys have a clue on what was actually going on in the area at the time that got so distorted into these maps and gazetteers of "Biledulgerid"? Wiki's articles on Algerian history focus on the coast and the articles on Mauritanian history literally skip from the Romans to French colonization.
(Sorry for the ugly formatting: I'm unsure how to create a thumbnail here that links through to the full images. If any of you do know, please do edit these.)