We use 12530 photometrically-selected BHB stars from Sloan Digital Sky Survey DR7 to estimate, the total extinction of the Milky Way in high Galactic latitude, RV and AV in each line of sight. A Bayesian method is developed to estimate the reddening values in the given lines of sight. Based on the most likely values of reddening in multiple colors, we are able to derive the values of RV and AV. We select 94 zero-reddened BHB stars from 7 globular cluster as the template. The reddening in the 4 SDSS colors for the northern Galactic cap are estimated by comparing the field BHB stars with the template stars.