The effect of the heavy b-quark mass is studied measuring the normalized n-jet rates of b-quarks with respect to light-quarks (l = uds), Rnbl (n=2–5), using the Cambridge jet clustering algorithm with LEP data collected by the Delphi experiment at the Z peak. Results are compared with generators (at hadron level) and massive LO calculations. R3bl is used to extract values for the b-quark mass at the MZ energy scale defined in the MS scheme, mb(MZ) and to test αs universality. The validity of approximating massive NLO corrections by their corresponding massless ones is investigated with the aim to measure the b-quark mass in four-jet events.