The Black Knight Greatsword is quite rare and isn't too bad... though I don't think it's worth for it for the stat requirements, the normal bk sword is better (at least to me). Though, I may be wrong, I never acquired it on my strength based character, nor upgraded it. (upon looking through the wiki, yeah I wouldn't say it's worth the 32 strength needed to use it)
At least on your first playthrough, I wouldn't worry about it too much.