While flying at 1500 feet, the static source gets blocked. When the airplane starts decending, what will the VSI read?

When the static source is blocked, the VSI cannot sense pressure changes in the static port. The VSI requires a pressure differential between the static port and its calibrated leak to indicate vertical speed. With a blocked static source, this differential cannot be established, so the VSI remains at zero regardless of actual descent rate. This is a critical instrument failure to recognize.