Do blanking intervals appear on both sides of a frame or just the top and left?
They are both right, depending on the context and level of detail. The latter accurately describes what happens within the waveform. If you look further down in your reference, it shows that there is a "front porch" and "back porch" of the horizontal and vertical intervals.

However, the former describes what happens practically, especially in terms of the useful parts of the vertical blanking interval (VBI). Closed captioning, teletext, VITS, VITC, AMOL all use the blanked lines at the top of the field to transmit their data. (You should also note that your reference intentionally chose this layout in order to draw parallels with digital VANC and HANC, which are only at the top and left of the picture.)