 Note that 0 IRE is equivalent to 0 V. The black level is equivalent to 53.57 mV (7.5 IRE)

The PAL video signal is slightly different in that it swings from -300 mV to +700 mV, instead.Thus, one PAL IRE unit is 7mV, where -43 IRE is equivalent to -300 mV at the sync tip, and +100 IRE is equivalent to +700 mV at the peak video level.Black level is the same as the blanking level 0 mv (0 IRE) 

The reason IRE is a relative measurement (percent) is because a video signal may be any amplitude.