the biggest reason people can't get their DS to recognize blue is background noise. if you play in a quiet room it will read you fine. if you are playing in the car or something, you will have a very difficult time no matter how you say it.
It recognizes my voice perfectly fine... you gotta make sure you're not breating heavily and say in a clear, consistant voice. I think it's missed my voice twice so far for the seven or 8 sessions (20 minute each) I've play so far.
I don't have any problems with it recognizing my writing expect my 'k's and my and '5's.
My DS has no problem recognizing my "blue", but it does have problem reading my 2 friends' and my sister's "blue". And I just say it normally, like, blue, at a faster than normal speaking pace. If you drag it out, like emphasizing the B and L, it's not going to work that well.