I find blanks to be something beneath the attention of the Dark Kin. Think about it - they're some lowly mon-keigh that make psyker heads hurt while a regular denizen of Commorragh might hardly notice anything special about them. I could see them as something of a pet for an eccentric Archon or two, a decorated slave with a nifty ability that he can sometimes use but little else.
I think the haemonculi have plenty of vicious creatures and un-creatures in their arsenal that a human blank might just seem a bit boring. Why send a blank at a captured psyker, when a hive of psychneuein could do the same AND have the poor psyker scream in pain and terror as an invisible hive of ghost bees lay eggs in his brain. At the same time, that's where the excitement could come from - a mundane little human slave causing so much suffering to a psyker just by offering him food and drink. Unfortunately, due to the nature of the Dark City, psykers within it are a rare sight and therefore there is simply little use for blanks in such an environment.
I have no problems in seeing a blank character appear in someone's background or story, though. The Dark City is big and there's a conosseur for any shade of unusual to the point that the most mundane of things could be seen as the most exotic. With the haemonculi, the sky is the limit (and sometimes exceeded!) to your imagination.