This question is vague, but I'll try to help. First off, don't just modify the blue sepearation. That will shift everything toward blue, including gray pixels, and I don't think that's your intent, so ignore Jan's suggestion above.
Color images are often rgb, that is img(:,:,1) is the red, img(:,:,2) is green, and img(:,:,3) is blue. By 'blue color alone', I imagine you are wanting to adjust pixels that are predominantly blue to become 'more so'.
Note: crudly, r+g+b = the gray component, cyan = g+b, magenta=r+b, and yellow=red+green
% get components r=img(:,:,1); g=img(:,:,2); b= img(:,:,3);
% When there is more blue than anything else, find out by how much extra_blue = max(b-max(r,g),0);
Now, you can try to add additional blueish component, as long as you stay in the valid range.
b_mod = min(b + k*extra_blue),1); % k is a scale of the amount of extra blue to add, say 0.1 for example
Alternatively, you can add the extra blue and scale down the non-blue components. Reduce r, g, and (b-extra_blue) by the exact same scale factor (say 0.9), that will maintain gray and all hue angles, then add a multiple of the excess blue (say 1.1) to the b channel.
There are much better ways, but then I'd have to ask you about hue angle, saturation, and it gets complicated quickly... I've said enough. I hope this helps.
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