How to read it
Each obstruction is ray-cast from the reference point to build a skyline: at every compass bearing, the nearest blocking edge sets an apparent altitude (arctan of height ÷ distance). On the dome that skyline becomes a shaded band hugging the horizon — the sky you can't see. Wherever a sun-path curve dips into the shade, the sun is blocked at that date and time; the marker dot turns red when the obstruction hides the sun for the date/time you've set.
Whether that's good or bad depends on season, exactly as Brown & DeKay frame it: blocking the summer sun is an asset (Strategy 27), but blocking winter sun is a liability for passive heating and daylight. The "% of sky obscured" figure approximates the loss of diffuse daylight (the sky component), so read it alongside the Daylight Availability and Sky Cover tools.