  We investigate the global properties of the radiation emitted by the
accretion disk around Kerr black holes. Using the Kerr blackbody (KERRBB)
numerical model, we build an analytic approximation of the disk emission
features focusing on the pattern of the produced radiation as a function of the
black hole spin, mass, accretion rate and viewing angle. The assumption of
having a geometrically thin disk limits our analysis to systems emitting below
~0.3 of the Eddington luminosity. We apply this analytical model to four
blazars (whose jets are pointing at us) at high redshift that show clear
signatures of disk emission. For them, we derive the black hole masses as a
function of spin. If these jetted sources are powered by the black hole
rotation, they must have high spin values, further constraining their masses.
