The blanking signal goes effectively through the Z axis amplifier (of which R1118 is the output), and proceeds to the DC restorer, that offsets the amplifier's output voltage down to more negative than the cathode voltage.
So, since your oscilloscope DOES have trace blanking, you must be wrong about the flat 11 volts on R1118. There MUST be a signal there, and its peak to peak voltage MUST be changing with the Intensity, otherwise the trace intensity would not be responsive.
Mind that, if you're measuring this signal with the very same oscilloscope (and not with a "test" oscilloscope as the calibration calls for), you will not see a signal, because the bottom part of the waveform is happening during retrace and therefore, will be blanked. But even not seeing a signal, you should still see a variable level (that varies along with the intensity).