BLU is the closest planet to SOL. Named Blution in the Mission Objectives of Mission 28, it is a deep blue colored planet, explored by the Seismometer Experiment in that mission. It has a strong gravitational pull which can make landing attempts somewhat difficult so matching your velocity with the planets spin is crucial to have a safe landing.
From BLU (+9) you can reach PUR (895), SOL (818), and GOL (965).
The radius of the planet has been calculated to be about 4 NaviComp units.
The rotational period has been calculated to be 24 seconds.
It is one of the two planets to be explicitly named in-game, the other being HOM, being referred in a mission as "Home planet".