Blanking
Blanking is the cutting of electronic current from the cathode to the screen in a CRT television while it is switched on.
Vertical blanking occurs when the electron beam reaches the bottom-right of the screen and needs to be reset to the top-left again. The VBLANK signal is generated by S-PPU2 (pin 26), which is fed into the S-CPU through pin 44.
Horizontal blanking occurs when the electron beam reaches the end of a scanline and needs to be reset to the beginning of the next scanline. The HBLANK signal is also generated by S-PPU2 (pin 25), which is fed into the S-CPU through pin 43.
Forced blanking occurs when the PPU nearly switched off, so it is generating black, but this is not part of the TV's sync pulse. It is believed that when disabling forced blanking, the PPU may produce garbage output until the next scanline because the color values in the latches are stale. [1]