From Intellivision Wiki
The VBlank Interval (short for "Vertical Blanking Interval") is the period of time between when the STIC signals a VBlank Interrupt and when it starts generating an active display. Programmers must be aware of this time interval, as it is the only time when the CPU may access the STIC's registers, the Graphics RAM and the Graphics ROM.
VBlank Period 1 defines the time window within the VBlank Interval where the CPU may access the STIC's registers. VBlank Period 2 defines the time window where the CPU may access Graphics RAM and Graphics ROM.