Well, here's my therioe on the matter.
Riku is almost definitly the Blind-folded unknown. He isn't the Dual-Wielder because his powers are based on his heart's darkness, whilst Keyblade masters need pure and strong hearts. The keyblade is a weapon of light, and it has already rejected Riku. Twice.
Further support of Riku being the Blind-Folded Unknown lies in his dark powers. I disagree that light hurts Riku, since he also has control of his light. However, his darkness is his true source of power, and he has developed sufficiently to spawn a few dark powers ("Dark Riku", identification by "scent"). Now, if one dwells in the darkness, while still have a good view of the light,he generally sees things better than those who stand in the light, dazzling themselves. Therefore, Riku has probably developed to the point where he sees better, since when in total darkness, his sixth sense kicks in, allowing him to precieve not what one precieves, but what really is. Therefore, his dark powers would be hightened when in a state of purpetual darkness, ie, blindfolded.