BLU is an abbreviation for the colour code "Blue".
Colour codes are issued by UK Military fields and are shorthand for the cloud and visibility prevailing at the time.
Blue is shorthand for 8K or more vis and ceiling greater than 2500
Decreasing ceilings and/or vis are represented by the descending colour series WHITE, GREEN, YELLOW, ORANGE, and RED.
The additional colour BLACK indicates that the field is closed for not wx reasons.
Hope this helps.
2D