All of the BHB stars used in this work in the color–color space. The tails of the black points are due to the extinction. The red points are the 94 template BHB stars calibrated with the known extinction. The magenta arrows show the reddening direction, the starting point of each arrow is the median value of the color indexes of the 94 template stars, and the terminal is the reddened original point with R_V = 3.1 and E(B − V) = 0.1.