The Black Mountains are a small range of mountains located on Aurora Island, believed to be the link between the Papas Alps of the Northern Continent and the Torles Mountains of Bubalook. They are definitely not as foreboding as they sound; their name comes from the fact that they are mostly covered in black walnut and ash trees.
The Searchers briefly skirt these mountains in Episodes 5 through 8 of the anime series, stopping at the cities of Poannka and Aurora, and the village of Yukiran. Along the way they meet Tiger and Hare.
There is a road through these mountains that is mentioned in My Monster Rancher as a place to battle (possibly the same place the Searchers meet Hare).