I found this on a site:

http://eyesonff.com/ff7/items.shtml#key

Check the key items(key item black materia)
Is this possible ?

Keeping the black materia !

**SEMI SPOILER**



You can't keep the black mataria.
You have it under your keyitem collection from the point you pick it up at the temple of ancients untill Cloud 'hands' it over to Sephiroth in the Northern Crater.

But technically, yes it's possible.... You have black mataria as a keyitem I believe, but only temporarly.

Shit !

It would be cool...

thanks

*SPOILER*

You dont have the Black Materia longer than 5 minutes or so, because Sephiroth manipulates Clouds mind to give him the black materia and so on. I'm not going to say much for those who havent played this far in the game yet.

*spoilers*

you have the black materia as a key item but try as hard as you want... you cant use it....
yes... i tried... tho like the other ppl said... you only keep it for a little while....