Is the blue square an imported image, or did you use the fill tool from within the image editor?
If it's an imported image, what colorspace did you use?
Does it look the different in every browser?
I just did a quick test, and I cannot duplicate the exact colour difference that you have. There will always be a slight variation in colour when rendering in a browser, but that seems way off. I even took a screenshot of your 2 blues and used those in my tests, and the results weren't that noticeable.
Perhaps you could try a slightly different blue, and see that makes a difference. Even just a few small tweaks to the colour may fix the issue. It could just be one of those freak colours that renders wildly different.
Also, your monitor might be enhancing the issue. On my IPS your colour comparisons look much closer in shade than they do on my TN monitor. So that might be partially to blame as well.