Hollow World

The "Otherworld" is the secondary setting of the story of Black Rock Shooter (at least in huke's artwork and the OVA version). Black Rock Shooter and other characters live in this realm, and the bizarre landscape is a witness to constant fighting between its denizens.

The true name for this realm is not yet disclosed, and to aid in reference, will be called "Otherworld". It is possible that this is the world of "Hazama" In BRS Innocent soul.

In the Black★Rock Shooter anime, it is heavily implied to be the "world of many colors" mentioned in Li'l Bird Li'l Bird Colorful Colors.

Black★Gold Saw's Battlefield
Shown in the OVA. A different version is seen in the anime.

In the OVA, it is only shown to be a large expanse of land covered by grotesque land features surrounded by an endless sea. From inside, it is composed of a variety of structures, with most buildings bearing a dilapidated, post-apocalyptic, and even desolate look in it. A striking feature of the structures erected in this realm is its dominant theme of checkered tiles in most of the floors and walls depicted.

In the anime, it seems to have ruined Greek-style columns, and many chains. Black★Gold Saw created the strange hooded figures here.

Dead Master's Lair
Shown in the OVA. Dead Master stays inside a structure similar to a cathedral complex located in the middle of a large depression, surrounded with winding passageways from outside. The area is surrounded by a large cemetery, which is shrouded in a thick mist. In the anime, the strange place where the Dead Master was chained might be her lair. In that place, there lies a hidden throne where the Dead Master sits for the duration of time when BRS is fighting her skeletal minions.

Desert Area
Shown in the OVA. A vast expanse of dry land with towering spires of earth and rock eroded over time. Strength was seen here.

Empty Area
Shown in the anime.

Black★Rock Shooter starts here at the beginning of the anime, although it is unknown whether she resides here or if she is only passing by.

The area matches the decor of the Otherworld in general and has pointed, deserted stones everywhere.

Chariot's Land
Shown in the anime.

Chariot seems to reside here. The area is full of robotic, toy-like creatures that have claws and drills. The background resembles a toy area with baby doll faces and colored blocks.

Inside is a house that floats in the air, suspended by strange string-like objects. Dead Master was dragged here by what was presumably Chariot's minion bugs and chained. Only a single doll covered this up.

Chariot and Black★Rock Shooter fought here until Black★Gold Saw intervened and made a fissure, separating Dead Master from Black★Rock Shooter.

Unknown Battlefield 

In this area, BGS is seen fighting with a berserked I.BRS. This area is filled with spikes and cannon-like objects, possibly referring to I.BRS herself. This world had purple-tinted surroundings. It is first seen in Mato's dream.

Trivia

 * While in the 2012 anime, the Otherworld is primarily referred to in terms of "the other world" or "that other world", the website occasionally uses the term "imaginary world" (虚の世界 kyo no sekai). However, since it always appears in quotes, it can be interpreted as a temporary name, and thus the true name of this world remains undetermined.