In every single Dr. Who episode there's been (from the original series, to the new series which started over here in 2005, of which we've just had our third), the Doctor's TARDIS is a 1950's-style London Police Box. Because of the fact that the chameleon circuits in that particular TARDIS don't work, it can never change its shape, and so always has been, and always will be, the blue police box.