Kind of embarrasing question...

[Nerfix]

This is strange.

[JaydedOne]

The blood slaves were equipped to your pretender?

Try dumping them back to the lab and casting the ritual that way. I don't believe (though I could be wrong) that you can cast ritual spells with components in individual inventories.

[UninspiredName]

Oh, that would explain it. I didn't actually do anything with the slaves after the Blood Hunt. I'll go try that out, thanks!

[Shovah32]

Yes, you dont equip the gems/slaves to the mage to cast a spell. Easy mistake to make though.

[JaydedOne]

And welcome, btw.

[Archonsod]

IIRC the blood slaves need to be in the lab stockpile rather than the pretender to cast a ritual.

The pretender's inventory is only ever used for spells cast in actual battle. Anything cast from the strategic screen uses the lab stockpile.