The TARDIS has actually been several types of blue over the years.
The real-world reason for this is that different models have been used for the TARDIS exterior. In addition to this, various models were periodically repainted, resulting in, as one example, the 80's TARDIS going from a light blue to a very dark blue.
In the show, it's often described just as "TARDIS blue" (f.e. envelopes in "The Impossible Astronaut")