First off, I have been contemplating on this answer for months. The below answer is the best I can do.

According to the model of [Hering][1], contrast in trichromatic species (such as most humans) is basically established between three sets of opponent systems: yellow-blue, red-green and the achromatic channel (dark-bright), and depicted in Fig. 1. The [opponent system][1] is established in the brain by cells in the [visual cortex][2] responding to both colors of a pair, but one color (e.g. blue) excites and the other (yellow) inhibits the cell, or *vice versa*. These systems are basically established by coupling the primary sensory neurons (the photoreceptive rods and cones) [(Gouras, 2009)][5].

[![!\[Hering][3]][3]  
<sup>Fig. 1. Hering model of color opponency. source: [Mark Green][1]</sup>

Because of this, the primary color-pairs yellow-blue and red-green yield excellent contrast, because in the brain the opponent response of cells will cause the edge to be sharply defined. Likewise, the achromatic channel yields high-contrast acuity. Mixed colors will impinge on this system not in an ON/OFF way, but in a gradient. For example, in your example the blue-side of the yellow-blue channel will be activated and contrasted against the achromatic channel. The highest possible contrast would be delivered the blue chalk with a yellow opponent system (use a yellow 'white' board :-). Now contrast has to be formed using the yellow-blue and achromatic channels that are phsyiologically not opponents.   

Another reason is foveal tritanopia: the fovea has the highest resolution for perceiving fine detail. There are no short wave cones (blue cones) in the very center, the area of maximum resolution [(Williams *et al*., 1981)][4], so presumably it is impossible to see blue with the area of the retina with the highest contrast acuity.

<sub>**References**  
**-** [Gouras, Color Vision. In: *Webvision. The Organization of the Retina and Visual System* (2009)][5]  
**-** [Williams *et al*., *Vis Res* (1981) **21**(9): 1341–56][4]</sub>


  [1]: http://www.visualexpert.com/FAQ/Part1/cfaqPart1.html
  [2]: http://isle.hanover.edu/Ch06Color/Ch06OpponentCells.html
  [3]: https://i.sstatic.net/ls6Ub.png
  [4]: http://www.sciencedirect.com/science/article/pii/0042698981902418
  [5]: http://webvision.med.utah.edu/book/part-vii-color-vision/color-vision/