This is a good match. Mid-low regen from Hellboy can handle Leonardo's sword given time, but Hellboy has much more experience, skill, and intelligence then Leonardo in the first place, and he has his own sword. He can keep Leonardo at bay with his pistols.
Hellboy just has a general advantage, so i'm voting for him.
Leo has great skill, and more self control than Hellboy. Too bad outright overpowering him isn't as feasible as this match up seems. I have no reason side with one or the other. However bloodlust being a factor, hard to say. Hellboy fights characters stronger than Leo and wins. Even faster ones than him. Experience alone plus the fact Leo isn't himself.