Why is the color index less than zero for stars that are hotter than stars of spectral type AO, and more than zero for less hot stars?

The color index B – V for stars of spectral type AO (T = 10,000 K) is taken as zero. Hotter stars, whose maximum radiation is shifted to the short-wavelength region of the spectrum, have (B-V) <0, while cooler stars, which mainly emit in the infrared, have (B-V)> 0.