I was wondering why Mickey Mouse, a denzien of light, wields a keyblade from the realm of darkness?

Because he found the it when he was wondering around in the Realm of Darkness. Plus, since he's a keyblade weilder he has access to using any type of keyblade. Weather being a keyblade made of darkness or a keyblade made of light.

He needed it for when he was closing the Door to Darkness. The keyblade of the realm of light and darkness were needed, so Mickey took up using the Kingdom key D and kept it, as a sign that he also believed that darkness could work with light hand in hand.

He had to get a keyblade from the realm of darkness. He needed the opposite of Sora's keyblade. Since Sora had the light keyblade, Mickey needed the dark one in order for the DtD to be closed successfully.

As others have said, he needed a keyblade from the realm of darkness to close the door to darkness. Also:

 

Keyblades have 2 different constitutions, either from the Light side or the Dark side. This is merely the exterior key, the inner key can act from either side and its abilities won't change. There is no clear division between good and evil. Usually the door to each world is closed and the keyhole itself is not visible. By bringing a keyblade nearby, the keyhole appears. But the lock is not actually closed itself. Indeed, the Heartless were able to find the door and easily open it and in this way steal hearts. In the previous game, Sora turned the locks and closed these doors. Furthermore, about the opened door of the integrated heart of worlds "Kingdom Hearts" itself, to prevent the outflow of the darkness within hearts the intent and reasons were different so the keyblade was used for a different purpose. In order to work, the door needs to be closed from both the Dark side and the Light side and both a dark and light realm keyblade are needed. The King needed a keyblade opposite from the realm of light keyblade that Sora had, so he went to the realm of darkness to obtain one.