What instrument indication suggests a blocked static vent during a climb?

During a climb, a blocked static vent primarily affects the vertical speed indicator. This instrument relies on the static pressure from the atmosphere to determine the rate of climb or descent. If the static vent is blocked, the pressure inside the instrument will not change as the aircraft climbs, leading the vertical speed indicator to show a reading of zero, regardless of the actual climb or descent the aircraft is experiencing.

In contrast, although other instruments might exhibit erratic behavior under certain conditions, the unique characteristic of the vertical speed indicator is to indicate no change when the static source is obstructed. The altimeter may also be affected, but it would not necessarily display a zero reading; it can show erroneous values instead. Therefore, the vertical speed indicator becoming stuck or reading zero is the most definitive indication that the static vent is obstructed during a climb.