Синий/голубой цвет значит что трекинг самолёта идёт через спутник.
https://www.flightradar24.com/
Why are some aircraft shown as blue on the map?
Aircraft displayed as blue icons are currently being tracked via satellite. Satellites are collecting the ADS-B signals from aircraft and transmitting them to the Flightradar24 network. Space-based ADS-B tracking will be used to supplement our terrestrial receiver network.