Originally posted by Fegee:

You need the bubble to break in order to increase the blood magic skill. You can improve this by giving your skeleton summons a bubble and have them engage in combat. Staff of Protection also increases blood magic skill for other players if you give them it as well.

Aaaaaaaaaaaaaah...

Okay, thanks. Suddenly makes a lot more sense, lol. Was worried it's not tied to any skill, mainly because the shield has a fixed value of hp, and does not incrementally increase via skill.

Happy that's not the case :)