In fact, I see no 'blank' signal in the video connector, so I'm not sure of what it is precisely.
Atari Monitor Connector
This connector carry out the video signal as well as the audio in and out and a general purpose output pin.
1 Audio out: This pin is connected to the Atari sound output amplifier. It normally goes to the Atari monitor sound system, however if necessary it is possible to connect a 600 Ohms headphone on this output.
2 Composite synchronization: As the name indicates it is a combination of the V-Sync and H-Sync signals.
3 General Purpose Output: The GPO pin is connected to the pin 6 of the internal sound chip YM-2149 and can be freely programmed by applications. Usually not used
4 Monochrome detect: This pin is connected to pin 17 of the MFP68901 circuit and is used to set the Atari in Hi-resolution when grounded and to Med/Low-resolution when left open. A transition on this pin generate a reset of the system
5 Audio in: An audio signal on this pin is mixed with the Atari internally generated sound and the resulting signal is sent to the Audio output pin 1. Usually not used.
6 Green: This pin is connected to the green analog output coming from the shifter
7 Red: This pin is connected to the red analog output coming from the shifter
8 12V / 10mA pins. On 520STF this pin is actually connected to GND
9 Horizontal synchronization coming internally from the Glue chip
10 Blue: This pin is connected to the red analog output coming from the shifter
11 Monochrome: This pin is connected to the monochrome output coming from the shifter
12 Vertical synchronization coming internally from the Glue chip
13 Ground http://jlgconsult.pagesperso-orange.fr/Atari/interfaces/interfaces_en.htm#Atari_Monitor_Connector