Access to each separate channel in OpenCV

You can use split function and then put zeros to the channels u want to ignore. This will result dispalying one channels out of three. See below..

For example:

Mat img, chans[3]; 
img = imread(.....);  //make sure its loaded with an image

//split the channels in order to manipulate them
split(img, chans);

//by default opencv put channels in BGR order , so in your situation you want to copy the first channel which is blue. Set green and red channels elements to zero.
chans[1]=Mat::zeros(img.rows, img.cols, CV_8UC1); // green channel is set to 0
chans[2]=Mat::zeros(img.rows, img.cols, CV_8UC1);// red channel is set to 0

//then merge them back
merge(chans, 3, img);

//display 
imshow("BLUE CHAN", img);
cvWaitKey();

In fact, if you just want to copy one of the channels or split the color image in 3 different channels, CvSplit() is more appropriate (I mean simple to use).

Mat img(5,5,CV_64FC3);
Mat ch1, ch2, ch3;
// "channels" is a vector of 3 Mat arrays:
vector<Mat> channels(3);
// split img:
split(img, channels);
// get the channels (dont forget they follow BGR order in OpenCV)
ch1 = channels[0];
ch2 = channels[1];
ch3 = channels[2];

There is a function called cvMixChannels. You'll need to see implementation in the source code, but I bet it is well optimized.