Achieving a reasonable SNR is vital for quality detection and measurement of stars during AF.
In post 147 of the thread Does SGP talk to Planewave’s PWI AutoFocus? Han says ASTAP uses a default setting of >30.
Blue stars in general are less common (though more luminous) than red, sensors are typically less sensitive to blue light, and blue light is more readily dissipated by atmospheric pollution, all of which I think make detection and measurement more prone to uncertainty/inaccuracy.
For what its worth I am currently using 15s exposures with B filter compared with 5s for L and 8s for R&G.
By way of compensation perhaps, I understand the human eye relies on Luminance rather than colour to assess sharpness in an image so I guess RGB focus accuracy is somewhat less important then L for seeing fine detail.