Note that this works for out of focus photos, not for enlarging tiny details of in focus photos. They may look similarly blurry, but are mathematically very different.
How's that? Can't you think of a pixel as the average value of it's subpixels (=blur)?
"Subpixels" are an artifact of the display type you're using. Although there may be individual colour components of an image pixel, they aren't restricted to a region of that pixel. And in most digital images - those made with a colour filter array, as opposed to a multi-chip, multi-shot, or Foveon-type sensor - the colours at each individual pixel are just an educated guess based on the surrounding pixel values; the RGB values are not captured separately, and attempting to increase resolution depending on which part of your screen is lit up by how much in order to render the colour that should appear at the corresponding pixel can only introduce artifacts. (If green is brighter than red in a particular pixel, it only means that green is brighter than red, not that the detail is laterally displaced.)