actually..
black: r=0 g=0 b=0..
white: r=255 g=255 b=255
for you to get a purple colour on a fade from blue to white.. you need to have some red mixed in.. so if you see purple when you mix blue and white.. you need an eye check

.. that and what your seeing is light blue.. not purple.. or maybe you don't know what purple is

..
now.. it's also understandable that it can apear purple-ish.. on some screens more than others.. especially with the background he has on his avatar (bad choise imo.. coz it does make it look a little bit like purple on some ppls screens i bet.. white is a strong strong mix of colours to have as background).. but technically.. there is no purple there

..