I just recently read a manga where there was a guy who turns into a black cat occasionally, can't remember anything else though. I think that it's a shoujo, hut I also searched the category cat boy with shoujo and none of those series were right, please help! Thank you

could it be Hakobune Hakusho... (though i'm not entire sure if there is such character... it has been long since i read it)
or might be Koi Neko (for the case of mikadzuki..)
but both are shounen and seinen..

Mugen Spiral maybe. It's not in the category I guess because he isn't really a shapeshifter but read the description.

Fruits Basket has a boy turning into a cat on occasion

It was mugen spiral, thank you

I also think it might be Mugen Spiral