The witchblade can be done with modular abilities easily enough.
Item of Power (witchblade) [-varies]
Discount (when not in use, it's a bracelet): [+1]
Modular abilities [-X]
And that's pretty much it. The X depends on the powerlevel of your game, but remember that you need to pay 2 refresh just to take the power. So if you want to have 4 points of refresh to shift around, you'll need to pay X=6 refresh for modular abilities, and the witchblade would cost 5 refresh then. You can add more modular points to the item as the character grows stronger. You could say the bond between the weapon and the wearer grows stronger, she learns how to better use it, etc.
After that, I would set up a number of configurations for the witchblade, which the player can choose to switch between. Let's say we've got the 4 refresh from above to play with:
Item of Power (witchblade) [-5]
Discount (when not in use, it's a bracelet): [+1]
Modular abilities [-6]
4 modular points
Sword:
Weapon: 3
Inhuman Strength [-2]
Inhuman Speed [-2]
Armor:
Superhuman Toughness [-4]
And so forth. The powers would change, depending on what form the Witchblade is currently taking. You could also just set up a list of allowed powers for the player to take, and she could choose them depending on the situation, and put the form she wants on it. A Sword form could, for example have different powers if you want to use it differently. Maybe you need it more to defend, so Superhuman Speed would make more sense than Inhuman Speed and Strength. Or you want to really hurt someone and take Superhuman Strength. Maybe you are just taking it out to scare someone, so Marked by Power and Incite Emotion might make sense. But ultimately, it all comes down to Modular Abilities.
If I was too confusing, feel free ask again, I'll gladly try to clarify things.
I do not have any idea about the darkness though.
