What you're describing is known as "foveal tritanopia," the "central blue-blind spot," or the "blue-blind zone." It's normally about 0.25 to 0.4 degrees of the field of vision (the full moon is about 0.5), corresponding to the part of the retina called the foveal umbo or foveal reflex. Most people are not aware of it for the same reason they don't see their blind spot. Blue light reflected from the central zone usually gets picked up by the nearest blue cones, providing cues to the visual system to adjust your color perception in the blue-free zone.
Some people (including a certain percentage of diabetics) have larger than average blue-free zones.
There are two reasons for the absence of blue cones. Different colors have different focal lengths, which matters more at the center where we have the highest visual acuity. The red and green cones have peak sensitivities that are pretty close together (530 and 557nm), but the blue cones are way down there in the low 400s. Also, the blue cones are bigger, while the red and green cones come in miniature versions in the center.
I think we all have this to some degree.
Computational Neuroscience: Trends in Research, 1997 - Google Books
Foveal tritanopia
diabetes
Extent of foveal tritanopia in diabetes mellitus -- Davies and Morland 87 (6): 742 -- British Journal of Ophthalmology
