Honestly, I think you could pull off either very well. You're probably thinking the blue wig wouldn't suit you because you're so accustomed to seeing yourself with black hair. Every time I put on a different colour wig, my initial reaction is "WTF I look terrible with this colour." But then I get used to it and it's not so bad.
Still, you don't have to go with bright blue for Miku - you could opt for a slightly darker shade and you'd still be very recognizable, or as Naiagu suggested, go for a different version of Miku (she's been rendered into a variety of different colour schemes).
But if you prefer Mio, you'd look great as her, too!