Riku says explicitly "consume" the darkness and return it to light. Which implies that he will take on the darkness and try to transform it into light by consuming and "cleaning" it.
He also says shortly after that part to Ansem: "Ansem! You're part of my heart now. Part of the light!"
That in essence means that Riku is going to do what is expected of him as one of the seven guardians of light (in a broader scale) and as one of the true Keyblade Masters (in a general purpose). Yen Sid explained that Keyblade Wielders are tasked with protecting the world of light since the conclusion of the Keyblade War. During that time the Keyblades were originally forged to do evil and try to conquer and enslave the light, however people then first used these weapons beyond their original purpose to protect the light after the war was over and nearly devastated everything.
It's clear that by the end of DDD Riku has fully mastered not only controlling his own darkness, but also developed a stunning resistance to it of which even Young Xehanort was impressed.
Riku may still use dark-based attacks like Dark Aura or Dark Firaga (they have become by now some of his signature moves), but only to further his services and exploits as a Keyblade Master to protect the Realm of light and the people close to him.
It's also somewhat natural, since Riku is also a being born in the Realm of Light (not to mention his Light is so strong as a kid that it attracted Terra from miles away, it shining just as much as Sora's) it's somewhat obvious that he would choose the light.
However, having mastered your own darkness is maybe one of the few ways you actually acquire the power to consume darkness and purify it into light.
With having such a strong resistance to darkness by now Riku can probably take on any darkness and be largely unaffected by it, focusing totally on battling it without having ever to worry about his own well-being anymore.
