TARDIS blue, also known as Scotland Yard blue, (AUDIO: The Magic Mousetrap) was a shade of blue that was used on the police box-shaped outer plasmic shell of the Doctor's TARDIS. Both Amy Pond and the Eleventh Doctor characterised the envelopes an older version of himself had sent as "TARDIS blue". (TV: The Impossible Astronaut, Day of the Moon)
However, the precise shade of blue varied as the TARDIS' exterior slightly changed across its long life. Indeed, the First Doctor noted a difference in colour between the Twelfth Doctor's TARDIS and his own. (TV: Twice Upon a Time)
Behind the scenes
"TARDIS blue" is a shade of blue with highly variable definition. As used in Day of the Moon, it clearly means "the colour of the Eleventh Doctor's TARDIS". However, as the TARDIS has been several different shades of blue over time, "TARDIS blue" is likely a relative term more than a specific one.
In the comic adaptation of the film Dr. Who and the Daleks, Dr. Who's own police box-shaped TARDIS was even coloured green as opposed to any shade of blue (although in all other media featuring this Doctor and his timeship, it was blue).