This kind of relates in a round about way.
I posted the below in a thread earlier today and didn't get any response. It would allow for what you're talking about...
Though the dagger is still considered a one-handed weapon skill, the perks for the weapon are housed under the stealth banner.
(From the Game Informer "Skyrim: Building Better Combat" article.)
So, this quote seems to work somewhat in contrast to traditional thoughts regarding the structure of perk trees in Skyrim. This seems to intimate that perks will be organized -- not under the context of skills -- but under the broader context of Combat, Magic, and Stealth in some way. Am I right? If that’s the case, I’m having some difficulty organizing the perk trees in my head, since this seems to indicate something a little more complex than my first thoughts.
Do you think weapon-type (i.e. long sword, short sword, dagger, war axe, etc.) are even perks at all? Does the character get better at
all one-handed weapons simultaneously regardless of whether they actually use them or not? And, then, the perk would not be "Dagger," but would rather be something like "Sneak Attack," which is a perk housed under Stealth and contingent on using a dagger from a hidden position, for example.
What do you think?
EDIT: I was probably a little lazy just copying this and pasting. To take it further, your "Blood Mage" example could simply be under the banner of Magic in a perk tree, as opposed to being directly related to a skill.