Black-blooded and Dual-Cursed both alter the Oracle's Curse feature though. It could be ruled that it wouldn't be legal. I'm not sure, since the Dual-Cursed Oracle doesn't have the "This replaces the Oracle's Curse" wording.
The tough problem would be deciding if the Black-Blooded would replace just one curse or both curses, which is why I think this wouldn't fly by rules as written. Too much ambiguity.
Yeah, I was just thinking about this myself for an NPC I am creating so googled to see if anyone else had asked this and found this discussion.
I am thinking it's really a cause of as Harley said, as written no as "Curse of Black Blood" replaces "Oracle's Curse" and Dual-Cursed is affecting "Oracle's Curse" not "Curse of Black Blood"
But, since Dual-Cursed doesn't say it replaces Oracle's Curse, it just adds another curse and takes away your Mystery bonus class skills, it COULD be interpreted as allowed.
In the end this is probably one of those run your concept by your GM... why do you need black blood AND another curse? Is it just to get the extra two revelations? Then no you can't do it. But if it really fits your concept, then I would allow it. Which, is what I really need to think about on my NPC... I like the idea of her having Black Blooded and tongues (Aquan) as a Sea Hag Changeling with the Waves mystery... but I need to decide if that's me justifying the two extra revelations or is it truly fitting to her conceptually. :)