Sorry for the late reply; I've been a bit tied up. There's no research about why blue bettas are more affected, although I've noticed that trend as well on these forums. I break down a paper on differences in immunity in red and blue betta over in my journal Fins & Things (Posts 12 & 26). Otherwise, just some potential reasons:
1. Blue bettas are genetically weaker/more inbred than red bettas, which are closer to the wild betta. Alternatively, selection for blue bettas somehow makes them more prone to "graphite disease".
2. Numerically speaking, there's more blue bettas in LFS/LPS than any other color. So if a betta gets infected, it's more likely to be a blue betta.
3. Maybe "graphite disease" looks/acts differently in red bettas since they have little iridescence, which is the outermost color layer and therefore also the most affected layer. My hypothesis is myco attacks from the outside in, so the most obvious color changes would occur on the outermost iridescence layer.