Yes, you can contact angels as a black magician. No, they don’t work for Yahweh so there is nothing for them to disobey. They don’t “switch sides.” They have their own work to do, and it is the same work as that of demons, to bring man to Godhood. They just go about it through different universal mechanisms.
Just remember though, that the feel of the energies are different and might feel off to you. As EA described in Evoking Eternity, after some heavy work with the demonic, he had need to call an angel, and the whole thing felt unnatural to him due to the difference in energies. You become more like those spirits you surround yourself with. However, the angel did show up, and it did fulfill the task so it obviously didn’t have an issue with EA being a black magiican.