If Web VTS is receiving AIS messages from a ship in 10 minute intervals, it will be displayed as a blue target. Web VTS displays a black target instead of a blue target when two conditions are met:
The ship has stopped
transmission of AIS messages for longer than 10 minutes
(i.e. Web VTS has not received any AIS messages from the ship within
the past 10 minutes)
The ship's speed was less than 0.5 knots when it stopped AIS transmission.
Displaying black targets enables position monitoring of ships even if they have stopped AIS transmission. Once a ship starts to transmit AIS messages, its symbol will change color to blue. By default, Web VTS will display black targets on screen for 6 hours.
If all targets in one area suddenly change color to black, it most likely indicates that your AIS receiver is malfunctioning, or that you have lost all AIS signals from this area.