How can I power the blue LEDs separetely on this aquarium light?
This is only an educated guess given the pictures and description. You need to do more testing and convince yourself that this works.

It looks like you could take wires 1 and 2 and connect them and get blue LEDs only, and 2 and 3 and get blue and white LEDs.  if this is true you could buy a little timer relay kit from any electronics hobby store and connect it between wire 3 and what wire 3 plugs in to.  Set the switch to "blue only" and leave it in that position, using the timer relay to do the white+blue cycle as you want.