Yes, they are weak to magic, however this does not mean you should use magic. There isn't a single spot in Old School Runescape where you can burst more than 5 at once. The size of the creature makes it hard to kill a lot at once.  Using ancient magic is going to be pretty fast, but it is going to cost you a lot of hard-earned GP. Even more so if you are going to use ice barrages. If you are going to use a single target spell like fire wave, it is still going to cost quite a bit, but it will be very slow to kill them. There are way better methods to kill black demons than using magic. So, even though they are weak to magic, I would advise you to use range or melee to kill them faster and in most cases cheaper.