I would personally use a Female Blood and a Male Royal due to the sizing difference. I'm not sure if a Female Royal would handle the eggs (I can only assume they would in fact be larger than standard Royal Eggs (I could be wrong, I'm just making assumptions))
As mentioned, from what I have read elsewhere, there is a degree of trickery sometimes needed. Putting the same species together, before switching them at the last minute (This requires a pair of both species, so both the male and female of the opposite species are in the 'mood')