I have started playing Skyrim again after being away from it for some time. I had replayed Oblivion recently and you can turn your soul gems black in Oblivion, but i cant remember if that is possible in Skyrim. I am playing on 360 so dont have access to mods, although i do have all the dlc. Is there a way to do this in skyrim? (I'm sure there is, I just dont remember how).

The only way I know of is to visit the Soul Cairn.

Use a Greater or Grand soul gem on lightning attractors in the Soul Cairn. Afaik you can do it only once, so it's not a reliable way of obtaining black gems. It's better to mine them in Blackreach or buy them.

Thanks i am currently playing through dawngard so it wont be too long before i reach te soul cairn. tis a shame there is no better way like using a transmutation spell or some such.