Why can't the human eye focus blue light?
The same thing happens in photography when you see an image with colorful shadows of objects on it - this is called chromatic aberration. Check this wiki page if you're not familiar with. Now this happens because the lens in the objective has different refractive properties for different wavelengths or if go the other way around different wavelengths refract differently within the same material. This is why the famous triangular rainbow prism is capable of separating the sunlight to rainbow. So back to the question: in our eye there is the eye lens that is responsible for focusing. Let's assume that it is homogenous enough. So when light enters into our lens it refracts to separate colours and our lens is not complex enough to fully compensate for this. The second picture is a good approximation of the human eye lens in shape.

Edit: so when you focus one color perfectly the others will be blurry a bit since those will be out of focus due to different refraction.



taken from: http://commons.wikimedia.org/wiki/File:Prism-rainbow-black.svg



taken from:http://en.wikipedia.org/wiki/Chromatic_aberration#/media/File:Chromatic_aberration_lens_diagram.svg