The Oblivion is made of darkness and light and colored black. Riku is consumed by the darkness, so the Oblivion resembles how Riku is.
Riku is seen with the Oblivion when Roxas fights him at Memory's Skyscraper in Kingdom Hearts 358/2 Days. He also wields the Oblivion when Sora fights him in Hollow Bastion in Kingdom Hearts 1.
DiZ needed Roxas to awaken Sora, so Riku awaited Roxas in Memory's Skyscraper. When Roxas fought and defeated Riku, Riku had to release his heart's darkness and assume Xehanort's Heartless' appearance to finally restrain and capture Roxas.
Riku
Soul Eater Sora's Kingdom Key Keyblade of People's Hearts Roxas's Oblivion Way to the Dawn Destiny's Embrace
Riku defeated Roxas at Memory's Skyscraper in The World That Never Was.
Sora&Roxas
Riku would most likely have tracked Roxas down, and finally released the darkness within him to stop Roxas. The only way that Roxas wouldn't have been stopped in the first place would be if Riku hadn't used the power of darkness at Memory's Skyscraper.
You're most likely referring to the Oblivion Keyblade, which is given to you by Riku (shortly before entering the castle at The World That Never Was).
No he did not. Riku gave into the darkness and defeated Roxas so that he could give him to DiZ (Ansem) and have his memories of the organization erased.
Riku and RoxasIn Kingdom Hearts 2, Riku fought and beated Roxas and brought him to DiZ to erase his memory of being in Organization XIII and to replace it with another memory, a fake one of Roxas having grown up in Twlight Town.
In 358/2 days Roxas can only dual wield it with Oathkeeper by putting 3 ability units on his zero gear in Mission Mode. In Story you will get an upgraded Kingdom Key. Giving Riku a Pandora's gear in Mission Mode will give him Oblivion.
Roxas sora and riku of course