Saïx opening a Corridor of Darkness for Roxas.
Corridors of Darkness are unpredictable pathways that interlink the worlds of the Kingdom Hearts universe. The Heartless use these extensively to reach other worlds, as do members of Organization XIII, their subordinate Nobodies, and many other characters. These pathways are formed out of Darkness, so it is unsafe for a normal person to travel them, because the constant use of these corridors would corrupt one's heart. However, the Heartless and the Nobodies don't have to worry about this, since the Heartless already have corrupted hearts, and the Nobodies do not have hearts to corrupt. However, one upside to this is that it provides a faster means of travel than the Gummi Ships.
Other characters who have used these corridors include Riku, Naminé, DiZ and even Pluto. Of these, only Riku, Naminé and DiZ have been responsible for summoning the portals, and only those lacking hearts to corrupt (Nobodies), and those who are darkness (Heartless) aren't affected, with two noted exceptions; According to Nomura in the Director's Secret Report XIII, Riku and Mickey aren't affected by darkness at all, due to some "secret similarities". Ironically, in the first Kingdom Hearts, after the faceoff in Neverland, Riku's heart was significantly weakened as he carelessly used a Corridor of Darkness to return to Hollow Bastion. Nomura goes further to state that DiZ and the Beast can successfully use the Corridors of Darkness, and perhaps open them, due to their particularly strong feelings or hatred. Furthermore, despite Sora having used the Corridors of Darkness several times before, he has not used them frequently enough to allow the Darkness to stain his heart, and the influential power of Darkness depends on the strength of a person's heart.
In Kingdom Hearts Re:Chain of Memories, whilst DiZ gives out black cloaks to King Mickey and Riku, he says the cloaks protect the wearer from the darkness.