In Jim Butcher's most recent interview with Daniel Greene, Jim mentions that in order to do magic you need both the study AND the bloodline. He also mentions that in order to be a Wizard, you need the bloodline whereas you are just a sorcerer otherwise. All about where you draw your power on I imagine.
This is interesting for several reasons.
There have been many references in the series about how powerful blood is, and particularly in magic. Blood of Christ is exceptionally strong as a curse for Michael, Harry's relation to Margaret Le Fay and eventually Eb is also played up a lot, mortal magic and blood having unique power and properties, blood sacrifices and bloodline curses and even death magic through a bloodline, not to mention that only people related to royalty can wield the Sword of the Cross.
It also (for me) highlights my theories around Harry's ancestry being important. Merlin was though to be the son of a women (sometimes a witch) and a succubus, or else the Devil himself (as sometimes Merlin was identified as the Antichrist) and his powers were the source of these interactions. I have speculated that if Harry is the Merlin, being the son of Lord Raith (a succubus) or even the Devil would add up. We also have the information that sometimes the scions of unions between mortals and the supernatural beings can look normal but have freakish power. This also could fit.
In Hellboy, it is Hellboy's mother who is really important. His father is Azazel, a Grand Duke of Hell, but his mother is the direct descendant of Mordred. This would also tie in with the King Arthur mythos (that is present subtly throughout the series). This ties Hellboy to being a direct descendant of Arthur and also Morgan Le Fay, able to be both the rightful King of England (mankind) and the witch Morgan (who was the apprentice of Merlin).
Harry at least is a descendant of Merlin through master to apprentice, as Ebenezer's journals go back all the way to Merlin. And often there is the theory that Merlin lived his life in reverse, perhaps Harry will too.
Harry has interesting links to Merlin too. Vadderung supposedly trained Merlin, and Harry has been helped by Dresden too. It wouldn't even totally surprise me if all three were one and the same. More than one person has held multiple identities.