Yes using pvc is legal, I have done so when I can't get to a certain spots in attics. You can only use 80% of the pvc. If your using sheathed electrical cable there is no way you will come close to that. Using THHN you might need to do some calculating. More than likely you really don't care about all that. The next question about the blue boxes. They need to have the same size holes as your pvc. Example if you have 1/2 pvc you will need a box with 1/2 knockouts. Use a 1/2 male adaptor to connect the pvc to the box. Tell you local hardware store about all this and they will get you lined out.